At our major local (US) festival (WVA at Winfield KS) lots of people plug their campers in through 100' "extension cords" with relatively few problems. For a 20 Ampere service, the "breaker" size at most of the places we can plug in, and for "light loads" you would need a minimum of AWG12 (B&S#12) 3-wire cord at 50 feet, and might get by with that at up to 100 feet if you're not running an air conditioner or other high-demand device. With 100 feet of AWG12 cord, a microwave is "iffy."

For a "US standard RV" 30 Ampere connection, you should have a minimum AWG10 cord and many RVs use #8; and the recommendation is that you should not exceed the 30 to 50 foot cord length usually supplied with the RV, although people at this festival commonly run their air conditioners on 100 to 200 foot AWG10 "drops."

The AWG10 is the heaviest "portable" cord commonly available in my part of the US, and a 100 foot "extension cord" can be had for about $90 (US) or a little less with some shopping. It's commonly used by construction workers, so should be available at lumber or building materials outlets. It should be noted that it's about 35 or 40 pounds of extension cord, and doesn't tuck into a corner of the trunk (boot) easily.

With UK 220 Volt mains to plug into, you should get by with a more compact cord, but voltage drop may still be a problem with common "household" or "yardwork" cords if you're operating at more than nominal load and beyond 50 feet or so.

Packaging on new extension cords should show an ampere rating for the length of cord in the package. If you can find a cord long enough for your use, and use shorter cords all of the same wire gage as the full length one, you should be fairly safe, although each connection adds a small bit of additional drop.

With UK 220 volt circuits, your 600W PA may have an actual peak input power requirement of about 1,000 Watts, so it should run at not more than about 4.5 Amps (peak). You can probably check the nameplate for a "Peak Amps" required.

Because of the lower voltage used in the US, the lightest wire size I have info handy for is a B&S12 (or AWG12) which should have a resistance (at 50C) of about 1.775 Ohms per 1,000 feet, so your 200 foot extension cord would have a nominal resistance of about 0.355 ohms per wire. At 4.5 amps, this would give a "drop" of 0.355*4.5 = 1.6 volts. Since both the "in" and "out" wires carry the same current, you'll lose about 3.2 volts between the input and output ends of the cord. You'll need to check the ratings on voltage tolerance for your amp to see if that's acceptable.

Factors that require special consideration:

1. If the cord is where any foot or vehicle traffic may cross it, you likely will want a more "robust" cord than is required strictly from the electrical capacity needed. You may also need a way of "tying the cord down" to prevent tripping, and to minimize wear on the cord if it gets trampled or kicked around. On a hard floor, you can perhaps "duct tape it down" if the facility permits it.

2. Although the 4.5 Amps x 0.71 Ohms reflects a dissipation of only a small bit of power in the cord itself, it can be enough to raise the cord temperature significantly, and "local resistances" in plugs or kinks can easily create hot spots. The resistance of the cord wires goes up rapidly with increasing temperature, which increases the rate of heating, which can lead to additional voltage drop due to the heating or to failure of the cord, especially if a bit of extra length is wound up in a pile so that heat is not easily dissipated. And note that "taping it down" holds the heat in too.

3. Any connector that's exposed can be rapidly burned out if any moisture gets in. Outdoors, it's often the morning dew. Indoors, it can be a klutz with a cup of cofee. Our campers sometimes "duct tape" the connectors, but a better procedure is to wrap the connectors with a baggie, and duct tape the baggie, when needed.

It's far more likely that the venue operators will impose limitations on what you can do than that there are regulations specifically prohibiting anything that will apply. If regulations do apply, the venue operators should know about them. (Or are you the venue operator?)

John