The exit interview is pretty straight forward and takes like 5 minutes. It just consists of a sheet that you tick and sign. You'd have to be completely disastrous for them to say you aren't suitable for any future re-employment.
As for the question regarding the discount card, it terminates when your contract terminates. If your contract is extended without any leave then you can carry on using your card. However, if you leave for a while, then decide to return you need another card, which is a nightmare! :/ given you have to wait for ages.
I've just left the Liverpool store as I'm moving to Manchester for Uni, so I have an interview/more of a discussion for the Trafford Centre store.