Is it possible to buy an unlocked Samsung in the US and bring it with you? If you can do this you can get a UK prepaid sim (cut it down if necessary, I regularly have to do that when I visit the US and want to use my iPhone). Or you might find a carrier that is prepared to put you on a contract.
Otherwise perhaps you could get your Blackberry unlocked? As you said, your contract ends one month before you move so the phone is paid for and your carrier should give you an unlock code. We did this after being with AT&T for 2 years and they were happy to give us the code, but if not you can get phones unlocked in the US/UK for a small charge.
Then, when you have the time, and money, you could look into getting the phone of your choice, on contract or prepaid, whichever suits you best.