The RSPCA will just have the cat put down if they cant find an owner. 63% of cats they take will be put down irrespective of health.
They seem to be on a power trip lately with lots of highly publicised court cases.
http://www.telegraph.co.uk/lifestyle/pets/10149908/The-RSPCA-made-US-feel-like-criminals.html

http://rspcanotwhattheyseem.blogspot.co.uk/

You can go the docs and legally kill a healthy unborn child but try that with your pet and you can land you in court