The above is totally wrong for London. London is totally different to other student cities.
If you want to live in London over the summer, look in late April/May to move in at the beginning of June. I took a look round a few houses at that sort of time to get an idea of what was about, but we didn't start seriously looking until mid July and the lease started on the 1st August. You can leave it a bit later but then you run the risk of not having it sorted out when you need to start back in September. There is no real concept of 'student housing' in London where landlords will arrange the next tenants months in advance; you're just another bunch of tenants to them. Also don't expect anything that some of your lucky friends in student cities might get like half rent over the summer - it just doesn't happen in London. They can easily fill the house with non-students who will move in straight away for full rent.