hey sunny,
i'll be glad to help as best i can. if you've been made an offer at UCL and plan on/or have already accepted you can download the application from the housing website and send that in. If you're waiting to send it in because you're not sure if you're going to UCL, the deadline for the postgraduate app is June 30th or thereabouts. i was told by housing that if you have a preference for a certain building to include a cover letter with your application stating your preference. they did say however, that this is not a guarantee that you will get your preference but since were some of the first to get allocated being graduate/international, i see no problem with us getting our preferences.
i sent in my application with a cover letter stating that i would like Hawkridge House. Its one of the cheapest but a bit far away, almost 2 miles, a 20 minute bus ridge or 10 minute bike ride. it comes highly recommended by a student living there at this very moment. it is mostly postgraduate and newly renovated. the building is set up as flats/apartments with up to 6 people sharing one flat. there are large kitchens and usually 2 of each, showers, toilets, sinks, etc.
other postgraduates can be placed in buildings like arthur tatersall and john tovall but are mixed in with undergrads. they are single rooms but are usually on a corridor with communal bathrooms/kitchens.
i guess it all depends on what kind of atmosphere you're looking for and how much money you've got to spend. i'm paying for school on my own and just finished my bachelors degree which means i haven't got a lot to spend, which is why i'm opting for hawkridge. its also in a residential part of town which is supposed to be more "london" than living closer to campus, which is mostly a undergrad/uni feel.
this is what i've gathered so far and if you have any other questions, feel free to ask. i'm sure i've left something important out.
also after you've sent in your application, you should get a postcard in the mail that says they've gotten everything and when to expect to hear from them. i think they put sept. 7 on everyones but if you're unconditional firm by may/june, then it seems like you get your assignment mid-july!