Yes, laing, please post your question on the London forum. Also, there is a 'sticky' at the top of the forum that gives you a couple of websites to consider when searching for accommodation. The TA London Hotels page lets you search by price range. Once you've narrowed down 2-3 choices, ask about them on the forum.
Also, if you do a search in the London forum search function for 'accommodation £100', I'm fairly certain it will direct you to various threads, as this question has come up before. (you can also search for 'how to make pound sign', as there was a thread on that a couple of months back).
I too look forward to reading about your report from Thirsk!