Ever since I settled into my wee little flat on Abbey Road last year (after a tough long distance search for a flat and a week long search after arriving), I have been hoping to help out the incoming class with searching for their own flat. You will soon hear about a flatcrawl where people get together to find roommates and see tons of posts from fellow admits looking for flats/flatmates. However, it is still pretty depressing sitting in another country trying to find a flat in London before you arrive.
Bias Alert: I only have information about some areas around the school. Many students live away from school and commute on the tube.
Tip: Don't break your head over it until you arrive. You will find a flat after you arrive. Calling agents and clicking through websites (listed below anyways) are futile because agents need to show you flats. "Viewing" as it is called here is important. It really is. You must see a flat before you sign up for it because it is really hard to tell how a flat is from a few pictures.
The picture is an attempt to mark out walking distances and a few popular building/blocks where students currently live.
St Johns wood is a popular post code. It is an expensive area but it is really pretty and safe. You will mostly be within a 20 minute walk to the school from here. Rents in the area are typically upwards of 200 for a studio and around 275-300 for a one bed. They can be much higher for better blocks.
Many students prefer living right across from the school in Ivor court, Rossmore court. These buildings are so close to school you can even catch the wireless network signal from school. Hanover gate masions is another popular block and is a 1 minute walk from school. There is a large number of students that choose to live towards Baker street station/Marylebone street which are both 5-10 minutes away from school.
The following is a list of agents that are located around school. Some of these agents waive the agent fees for students (you need a school letter for this). Rents can be negotiated upto 10% downwards. However your bargaining power diminishes as you get closer to the start of the term and flats are almost all gone.
JAC strattons - www.jacstrattons.com
Oakleys - www.oakleys.co.uk
Sandfords - www.sandfords.co.uk
KnightFrank - www.knightfrank.co.uk
Foxtons - www.foxtons.co.uk
BrianLack&Co - www.brianlack.co.uk
Benham Reeves - www.brlets.co.uk
General property search websites: www.findaproperty.co.uk, www.gumtree.com, www.loot.co.uk
Hope this helps cool your anxiety a bit.