If you've met your offer then congratulations! You'll have a fantastic time at UCL.
I've seen a lot of messages on here from people asking to swap their accommodation offer from one hall to another. Please do not waste your time.
swap lists do not open until October.
You cannot swap without going through the accommodation services as your personal details are recorded by the hall in which you've been allocated. You won't get away with an informal or unofficial swap.
Most people end up loving the hall they live in.
Other universities allow pre-move in swaps but (as you'll learn soon) UCL likes to do things differently (for better or worse...). Earlier exams, later graduation and unfortunately, no way of swapping accommodation before you move in.
I hope this helps clear things up.
... and the ones that won't