Firstly, try these two things.
1/ contact your local council's parks & recreation dep't and ask if they have any pools that you could use. my local councillwas more than helpful.
2/ contact British waterways and ask same question, they may well be able to identify a lake/canalbasin/ pool near to you that they dont mind you using.

* be aware that you will need 3rd party liability insurance when using any public accessed water stretch and both these bodies will ask you to provide documentation for.
I for example use MPBA [model power boat association] cover for our club when we formed it, its reasonably cheap, give all the cover any council would want [10 mill] and gives you access to a load more clubs too.

"pm" me for more assistance if you need it, no worries.

HI Colin,

I've been in Hereford for 12 years now and most suitable places I've found are fishing only, but you can sail real size boats up at Bodmin lake, near Queens woods as there is a boat club there, I have sailed my model yacht up there but haven't put the fireboat into the water, as its very deep and the access is difficult and to be honest I'm not sure I'm Allowed to be there? No one has complained "yet", about the sailboat, but remember half of the lake is a nature reserve and they may not take to powered boats.

Simon