Where can I book these on the internet?

I would really love to go to London, but I am worried that it might be a bit too late to book the hotel. I was going to stay home over my break from university, but friends are going to London and said I should come along. I would need to find a hotel room though, and I am not sure how easy it is to find last minute London hotels. Can anyone give me any advice on how to find a last minute room? Is there a good website I can use to book hotels at the last minute?