Bathrooom scales, as I've seen them, are very simple devices and ALL COMMONLY SOLD IN THE US have a small "button" or "knob" you can use to "zero" the reading with nothing on the scale, after which they are quite generally accurate within one or two pounds - within their rated range.

I would expect that UK scales would have the same kind of adjustment.

On some scales, especially cheaper ones, there is sufficient "stiction" to make the "zero" point ambiguous, so you need to push down (4 or 5 pounds) and release quickly, so that they can "wiggle their way" back to zero, before using the knob to make them say zero when they're at zero.

The majority of cheaper bathroom scales here are "rated" for a maximum of abour 250 pounds (115 kg?) although some have numbers beyond their rated range. If you weigh more than the rated capacity, the "spring" can be bent, rendering them less accurate, although within the range shown on the dial the error is fairly small. Higher capacity scales, up to about 500 pounds (225 kg?) are generally available although not all sellers stock them regularly.

"Electronic" scales can also sometimes be rendered useless by getting soaked with water, and some may have errors due to a dying battery (especially if the battery came with the scale and it was "on the shelf" for a long time).

A fairly mediocre "balance beam" scale (movable weights on a beam) eliminates the few likely sources of error in typical "spring scales," although they still need to be "zeroed" occasionally. If you're heavy enough to "bend the spring" your local veterinary supply house probably can get you a "small livestock scale" with capacity up to about 1,000 pounds (450 kg), although one of those will be significantly more expensive than the usual bathroom scale. Scales of this kind (in lower capacities) were commonly used in medical offices, but have been mostly replaced by "electronic" ones (less consistently accurate than the home bathroom scales IMO) so you might find a good one at an antique shop/junk store.

I don't have Jim's problem of seeing past the bump, but sometimes have to put my glasses on to see as far as floor level where the dial is. I blame the day-to-day vairations on the differences in my several pairs of eyeball stretchers.

John