You can submit a preference but thats all it is, a preference. You will get put where the navy needs you to be. In my experience classes usually get split 50/50ish between Plymouth and Portsmouth based ships, but thats not always the case, it can be any class of ship. After Phase 2, I think you now join an Engineering Training Squadron (ETS) in a base port to get qualified to ET1. So you'll be working on a harbour training ship, which could be T45 or T23. I might be wrong so don't hold me to that.