As with most things, if you pay more you will get more, and in London especially the cheap hotels aren't always very good. So if you are able to be flexible about when you travel, you may well get a better bargain by looking for a slightly higher priced hotel but trying to get it cheaper through a late deal. if you look at sites like lastminute.com you can often get a good break at a fair price.

Another way to save money while still getting fairly good quality is to compromise a little bit on location. Tourists usually want a central location, which is more expensive; but if you don't mind staying a bit further from the centre and getting an Oyster card, so you can use public transport more cheaply, you can often find somewhere very nice (as well as less crowded and less noisy.)

Another way to search is to put a phrase such as 'good budget hotels london' in a search engine. That way you can find more individual lists and articles that may help. You can also consult Tripadvisor, which is generally fairly reliable.