You can always apply, then decide you don't want to take it. You can move out any time within the first term without breaking the contract.
Definitely don't get to choose, sorry mate. Camden Halls are v rough, but there are advantages ie not living right on campus, cool area, more people in one hall. It's not really a bad area - it's scary at first but the worst I've had are several rastas trying to chat me up...I've come back in the middle of the night several times and I'v never been scared, mainly cause it's always so busy. The halls are a bit up the road, but there's always the bus.
Most of the halls are within max 10 mins from uni. Ifor, Max Rayne and the Lodge are 40 mins, but the buses are good, especially if you're lazy like me. And one's the 29, which is free unless a conducter guy gets on in which case you're screwed if you're not by an oyster card machiney thing. But that's another story.
Main advice: Whatever you do, DON'T tick the box that says "If the only place we can offer you is shared, would you take it?", unless you really don't mind/want to pay next to nothing. Everyone who ticked that last year got but in a double, regardless of putting that the single most important thing was a single room. I'm alright though, I scared my roomie away within a month.
Applying early probably helps too.
You can only choose between intercollegiate, pot luck which ucl ones you get into. Though I have a friend at Nutford House who says it's great.
And to the guy/girl who says they NEED a single, ensuite, self-catered room close to uni? Better put your prices high, mate. You're at uni in the middle of London - you don't really get to choose that much...