Bubbles, it appears very normal for private hall owners to have deals with specific Universities. A friend who manages private accomodation says it gives them access to University discipline procedures, ie you don't graduate if you have not paid your hall bills or have too many complaints filed against you, and it allows them to advertise via the university website and be part of the allocation process, and thus a guaranteed occupancy. KCL currently seems to be doing deals with the developers of every new block in central London presumably to be able to match "rivals" (UCL, LSE and Imperial) offer of guaranteed accommodation for all first year.
I suspect in London, Universities are also able to provide tenants for the summer from their lucrative summer schools. UG and PGs from Europe, the US, and Asia flock to London for a chance to spend four weeks in the capital, learn something interesting and perhaps improve their English. All of which gives Universities a handle to negotiate lower rents for their UGs.
Imperial have been developing a second campus at White City, elements of which are still being opened. My assumption was that early accomodation near here was difficult to fill, so was opened up to others, but as the University has expanded, and the area improved, this is no longer the case, or won't be when Crossrail (finally) opens and it is all more accessible. It could well be that it is now, or will be, exclusively Imperial.
Some, non University linked, private accommodation is very very expensive, though often quite swanky. My guess is that they have their eye on richer overseas students, and also single working professionals who want their own space and the benefits of a central location, a concierge and communal facilities.
And Framey, yes. As a landlord I have accepted 9/10 month fixed term tenancies from, say, overseas Masters students. Another difference in London is that for students rent is often payable in advance, as it is difficult for overseas students to find UK guarantors. Rent in advance is very attractive to landlords!
One problem with waiting till September in London is that you are then competing with new graduates starting well paid jobs. It is well worth starting two months early so in late June to find something available in late August.
Sorry, a bit of an essay on the London property market. At the moment there is not much money to be made in London BTL, but good returns from up-market hostels. (From employers needing to attract staff as well as Universities.) So that is the direction large investors are headed.