I think it depends on how you'dlike to see your money spent....
If its education and advice then I'd say support something like the RWA, or if you want to hit the "grass roots" then support a rescue, or sponsor an individual bun.
Before you donate to any charity though,I recommend that you look them up on the Charities Commission website to check that they are registered, and you can also see what they spend their money on, and how much they have in the bank - gives you an idea of which charities really need support.