Petsathome now have adoption centre's in most/all of thier stores. Make sure you talk to the staff to find out if they know what they're talking about though. I've found some of the staff to be negligent so I have gone to another store.
I just adopted two males who are 8 months old after thier previous owner took them into the store because he could not cope with them for whatever reason. The staff at the store were so nice and really knew thier stuff, so I was happy to adopt them. I also visited Wood Green Animal Shelter in Godmanchester but they only had a few females left. They have a website and several shelters so you could try phoning them before visiting to ask them if they have any?