Ok, I've decided what replacent phone I want. Sony Ericsson K810i. T-Mobile apparently sell it for GBP169.99, and everywhere else doesn't want to sell it on Pay As You Go. Play.com has it for GBP200 too, so I might just get it from Play if the T-Mobile shop doesn't have it in stock.
If I get it from Play, it's unlocked, otherwise it'll be locked to T-Mobile and I'll have to get it unlocked so I can use an O2 SIM (Since they're best value for money for more or less text-only, and I should be able to transfer my phone number).

So, Sod's Law dictates that the T-Mobile shop won't have it, and I'll end up getting a different phone.

In either case, I'll update this post later (maybe) if I get one.

EDIT: I suppose it helps if I know where I'm going before I try to go there. The shop isn't where I thought it was, and I can't be assed going back out now I know where it is. Oh well, tomorrow I guess...