I wouldn't donate to the RSPCA because of all the bad stories I hear about them, here, in the press and from friends who've tried to contact them for support. However, I would have no problems adopting an animal from one of their shelters and almost did last year. Those animals need homes and some may have a time limit on how long they're kept for before being put to sleep. I definitely wouldn't discriminate against those poor animals though I admittedly went with Cats Protection in the end (but that was because CP allowed me to reserve cats until after my holiday whereas the RSPCA wanted me to take the cat immediately - which wasn't feasible)
The RSPCA hospital in Putney was also fantastic with my elderly uncle's cat when she was ill, charging him just £100 for 4-5 days stay and treatment.