LovaLova - welcome to London!
You might get better value going nearer the 02 and travelling back into town when you need to then. The Aloft at London Excel is really nice and just over the river from the 02 and the Ramada Docklands (which is next door is good too - I've stayed at both), however, I would expect that given the location these might be full too!
Closest you'll get to your budget is probably http://www.premierinn.com/en/hotel/L...DCMP=KNC_D_BND
not as glam as the Marriot County Hall, but in the same building so the location is great! You're probably looking at £150 per night though (that's about as good as it gets for a hotel in central london you'll actually want to sleep in!