Hi - I work on Queen Elizabeth St and have seen a black and white cat a few times doing his rounds near Devon Mansions and also St Olaves Estate next to the park (I think thats what it is called) looked quite well cared for as if he lives near there. Might be worth checking for missing cat posters around the estate if you haven't already done that.
I'm not sure what RSPCA policy is on rehoming and euthanasia for strays but may be an idea to contact Cats Protection as they always foster and rehome.
Hope you find his owner
I can't recommend Celia Hammond enough if you guys think the cat needs to be rescued. I found a semi-feral cat last year on E&C roundabout - they were fantastic and went on to rehabilitate the cat. The RSPCA were as good as useless...