What address do you have with your bank account?
Right now I have statements go to a US address, however I talked with a friend and he said it was ok to change the statements so they go to his UK address. I am only afraid that it will be difficult (which I am still in the US) to get this done, i.e. they won't do it over the phone or let me fax a copy of my passport, they will mail a form which will take 7-10 days to get to the US then another 7-10 days to get back.
Once I get back to the UK I can change the address in person, but then it would take a while to get a statement and I need to use the phone ASAP when I get there esp. since I am looking for jobs, need update on e-mail, etc.
PAYG - as far as I recall - used to be much more expensive for calls and usage. For 20-25 a month on a month-to-month SIM only plan I can get 600 minutes, unlimited texts, and Blackberry data / email.
Finally, do make sure your unlocked blackberry uses the correct frequencies and mobile standards (formats) for the UK.
I have a Blackberry 8900 which I got through T-Mobile USA and is unlocked. It's non-3G phone so I am pretty confident it will work ok in the UK.