T-Mobile US or T-Mobile UK? I'm assuming you meant TMUSA since you said before you travel.
In that case, the answer is no. They do have Simple Choice plans with no contract that allow you to use international data for no additional charge (but data is limited to 128kbps), free SMS, and calls for $.20/min.
But if you are going to buy a SIM and get a new number, you may as well just pick up a SIM while you are in the UK. For £10-15 you can get a PAYG SIM with 100 minutes and 1GB+ of data, and it will be high speed instead of capped to EDGE speeds. Just make sure to look up the location of a mobile store before you go, otherwise you'll end up spending $20 on international data to find one in the Maps app!
I've used vodaphone, 3 UK and Orange before. 3 had the best tariffs and fastest speeds, but weakest network, just like T-Mobile USA, actually.