TB,
Can't help much re your specific venue (did all my exams at Gatwick) but in general hotels don't charge you to cancel if you cancel more than 24 hours ahead, so if you were using hotels, I would book at both venues and then cancel the one I didn't need.
Colleges might be different but ring them and ask what is the latest date you can cancel without penalty.
Good Luck with the exams !
QNH (not necessarily a wise one)