The Midlands Daystate Hunter comps over winter are also well attended.
Isn't there a list of clubs in the UK?
My own club is a small club and probably only has 15 members with others that come regularly as guests. I'd say about 5 or 6 shoot Worlds/ Nationals.
There will be a difference between how many shoot it regularly and over a sustained period of time and how many have had a go or pop in and out.