In the US, common household circuits are typically protected by 15 Amp circuit breakers, and at the 110 Volt nominal supply voltage the maximum power that can be used on a single circuit is approximately 1500 Watts (1.5 KW). A single circuit intended for "light appliances" may have a half dozen outlets for lamps and other low demand devices; but the entire circuit is limited to the 1.5 KW circuit breaker value. Most homes include a few circuits with 20 Amp circuits and in newer ones all circuits may be 20A ones, but in older homes they usually are installed only for specific uses such as "window air conditioners" or other higher demand appliances. Garage/workshops, even in older constructions, fairly frequently may have 20A circuits for the "power tools." The power limit for a 110V 20A circuit is still only ~2.2 KW, with few appliances showing higher than a 2 KW cutoff. Due to the common appearance of 15A circuits, the selection of available small appliances that exceed 1500 W is extremely limited here.

The "center neutral" configuration for US power permits relatively easy addition of a 220 Volt circuit, usually with 20A, 30A, or 50A breakers but in most cases each outlet will be a new installation specific to something like a stove, whole house air conditioner, electric clothes dryer, or the arc welder in the workshop. In principle, a 50A 220V supply circuit could provide ~11 KW of heating. Anything larger would require building permits and architectural and engineering consultants in most cases.

I've had insufficient exposure to UK and other practices to comment, but similar limits are likely to prevail. Perhaps someone can fill in the gaps(?).

At the 1.5 KW level, very small heaters are available here that will produce all the heat possible for <$40 (US). A typical, and very popular, one might be the Holmes "Cube." Similar ones are available from other makers.

(A single Holmes Cube - or my similar Emerson one - can warm my "barely insulated" 10 ft x 24 ft garage by 10 degrees or a little more, given a little time to come up. A pair of them likely could provide quite comfortable "room temperatures" down to around 30F outside air temps. I've only got two usable circuits in the garage so I haven't considered a third.)

These small units can be easily placed almost anywhere that an outlet is available, include a fan, a crude thermostat, overheat and tip-over protection. Disadvantages are that the element is hot enough to ignite a flammable object placed very close to the air outlet and being small they can be tipped over fairly easily. In nearly all cases the overheat switch will cut off the heater before "flames break out" but it's not too uncommon to see nearby items "heat damaged" where I've seen careless people use one.

The "cube" style heaters would be classed as "forced-air incandescant heaters." Older ones generally used wire coil heating elements, but some newer ones claim "ceramic heaters" that may be a little more durable. Efficiency is the same.

Larger units that work on the same principles can have the heating elements more fully enclosed, can use larger fans, and hence the output air is at higher volume but at lower temperature. The potential for better warm air distribution is obviously present, but the total amount of heat that can be distributed is exactly the same.

In the same small appliance groups, "radiant heaters" rely on the warming of objects in the room by the "heat rays" (infrared light) radiated by the heater. The heated objects will (theoretically) warm the air that surrounds them and the air will (theoretically) be circulated by convection. Since the conversion to "radiation" isn't perfect, the remainder of the input power is dissipated as heat within the unit, so some of these have a small fan to "blow the residual out" but that contributes no additional foom heating. A "radiant heater" can char or ignite objects placed too close to the "radiant surface" although the larger area of the "radiating surface" possibly makes this a little less likely than with an incandescant heater. Small units should of course include tip-over and overheat cutoffs. Most such units include a "thermostat" (more accurately a power setting control) but the huge lag between changes, before the effect is apparent in air temperatures, may make adjustment to your liking somewhat tedious.

The third kind of small heating appliance common here is the "electric radiator," or more correctly the "convection heater." Most such units are configured to visually resemble a hot water (steam) radiator, with enclosed heating elements and a convection circulated internal fluid that warms a fairly large external surface. Room air is warmed by contact with the surface and circulated via convection. Due to the large "warm surface" the temperatures at all exposed surfaces can be kept quite low, so there is little danger of burning anything, although some may get hot enough to melt a synthetic sweater or similar item(s) carelessly draped on them. Tip-over and overheat protections still should be included. This kind would probably be the "safest" for use in a more-or-less permanent meeting room, with the disadvantage that the time-lag to bring a room to comfortable temperature can be rather long, requiring the heater to be turned on long before the session starts, and increasing the possibility that it will be left on for longer periods, thus increasing operating costs. An auxiliary air circulation device, such as a ceiling fan perhaps, may be wanted with one of this kind, or with the "radiant heater" type above.

The maximum heating you can obtain from any of the three kinds of small heaters is exactly the same, since the limit is due to the circuit capacity and doesn't depend on what you plug into the socket. These smaller units are intended to be "portable" but for a dedicated use a clever person should be able to fasten one down at least semi-permanently and create a "safe zone" to assure adequate clearance from hot surfaces and air flows. A "stable" location is essential and recommended.

For most of these units, generically called "room heaters" or "area heaters," any thermostat will not be marked by temperature, but will be a "relative to maximum output" setting. A thermometer on an opposite wall, indicting in degrees, is a good idea, for reference.

Even in "deluxe" configurations, no heater of any of these kinds should cost more than about $100 (US) unless you're looking for a much more peramanent solution or have other things** that must be considered.

**[Prof: "If black objects are better radiators, why are radiators painted silver."
Bright Student: "Because it's prettier.]

Although a permanent installation probably isn't what you want, if a more sophisticated, or higher capacity, heater is wanted the appropriate devices would be in the class of "auxiliary furnaces" that would imply a permanent mounting, installation of a separate dedicated electrical circuit, a remote (wall) mounted control thermostat, and most likely a forced-air circulating system (or at least a fan). Here, most people would likely opt for a forced-air gas furnace of one kind or another (which may also require adding a gas supply line and an exhaust stack), due to operating economy. (The fuel used for basic heating should be considered, usually.)

Permanently or semi-permanently mounted gas or other fuel fired "room heaters" with forced air blowers are available in an intermediate range, sometimes called "space heaters," but require very careful consideration of venting and other factors for safe operation, and here such units are almost exclusively found (and marginally legal) in open detached garages/workshops. (A "safe" (?) installation of such units requires installation of a "chimney" for combustion gas removal and isolated combustion air inlets. Using room air for combustion air is legal only for small capacities as for water heaters in separately enclosed and isolated spaces. Some people take the risk of lesser attention to details with these units, but I'm easily frightened by them.) Electric units in this class are available, but the higher current requires a dedicated outlet and operating costs are sufficiently higher that they're seldom seen here.

Although UK (and other) wiring uses different voltages and presumedly different circuit protection levels than here, I would expect that "portable heaters" will have a power limit somewhere near the 1500 W found here. You are unlikely to find a "heater" with any higher capacity that doesn't require a dedicated (and likely new) circuit you're unlikely to have available. You should be able to start at the $20 (US) level and step up to whatever contributes to safety for your circumstances, but all heaters you're likely to find will be interchangeably equivalent in heating capacity regardless of how many "ad-features" and "look-goods" you also pay for.

(But the trick is to find the exception to the generalizations that does something for you.)

John