The Road Vehicles Lighting Regulations (1989) state that a rear fog light is mandatory. However, as long as it is adequately spaced away from a rear brake light and within a certain height band, there is pretty much free reign for the actual specs of the lamp.

The size, intensity and wattage are all not specified. For safety reasons, it wouldn't be prudent to go stupidly small but there are some quite powerful small LED packages out there. I shall research it some more, especially if my current LED cluster is starting to gradually fail.