Try going to the stores in person. That worked for me, in both Zavvi and The Sony Centre.
The Sony Centre being a franchise will probably have a different answer for every store. If the manager is half decent and has a fair amount of trust, he should let you on board, without even an interview.
Honestly, my week at The Sony Centre was really great. I know it's not strictly a games shop. I didn't really have to do much, it was rare they asked me to do anything, I mostly just stood and chatted most of the day. I still got to do the odd delivery/installation of T.V's - with the help of a full - time member of staff, of course. I guess that was kind of cool, in a weird way.
Besides, it's far harder for people to turn you down if you turn up in person. I still got turned down loads, but, it didn't take me TOO long.