You don't need (and, in fact, can't get) a pet passport outside of the EU. You will need to do the equivalent Third Country Official Certificate in order to bring your pet into the UK.
As far as going back, I got both of my guys pet passports, both as a security measure (it holds all their vaccination history so it's easy to hand over to boarding kennels as proof of up-to-date vaccines) and in case I want to travel outside the UK with them (or, at least, with the dog). So far, I haven't done it, but the passports were only about £50 each, so I figured it was worth it.
All you really need to do once you're here, though, is make sure the rabies stays up-to-date. I haven't brought an animal from the UK to the US, but it's much simpler than US to UK, so I don't think a pet passport would dramatically help matters.