I'm also on t-mobile PAYG and my £10/month top-up bonus is "unlimited" data and free national sms. The particularly nice thing about this whole "top-up" thing is that it really is a bonus - the £10 is sitting in your account to pay for calls and international sms and my up-to-£1/day-if-I-go-over on my "unlimited" data allotment. You just request a sim-card, pop it in your phone, put money on the account, and select your bonus!
As for phones, check and see if your current mobile can be unlocked and used in the UK.