If the phone is unlocked then you can use it however you will need to contact the customer support of US service provider, they will give you the unlock code (if phone is locked). Once it is unlocked then you can use any simcard.
As per the information above, make sure your phone is unlocked. Then, when you get to the UK, you can get a Pay As You Go SIM and top it up. You may want to opt into a Freebee - there is more info on these here
