Well, living in London means that I don't know much about hotels, but:http://www.londontourist.org/hotels.html
This site has some decent info in general, and I'd trust them quite a bit.
100 Euros a night isn't all that much when you're talking about staying in London - it can be very expensive here.
Alternatively, depending on how long you want to be here, renting an apartment may be a good idea: http://www.homefromhome.co.uk/about_us.htm
They're the first people I found on Google, and I'm sure there are plenty of others around.
Their prices seem fairly reasonable if you're sharing with other people. They're mostly very central and as a result I'm sure they'd be very nice.
Sorry I couldn't give you a straight answer, but I hope that helps!