you actually need to be taking donations of over £5000 to be able to register as a charity.
Legally speaking, Rescues don't exist. There is no framework, no registration of facilities or anything like that. and as you need £5000 to be a charity per year (which most rescues wont get) then you can't be registered that way either. so in essence, its extremely difficult to decide which are legitimate organisations and which are chancers. Word of mouth and reputation will be the best decider