Accommodation in London is very expensive. I have 2 suggestions; if you want to book in advance, stay somewhere like the Premier Travel Lodge - very reasonable price and good location (friends and family have stayed here and recommend it).
The travel Inn in question is the 'Travel Inn County Hall' which is the building adjacent to the London Eye. The nominal cost of a double room is £80 per night, and breakfast is an extra £7 each. I don't know if they have last minute stand-by deals.
See www.premiertravelinn.com or this specific page if it's still live,
http://www.premiertravelinn.com/pti/hotelInformation.do?hotelId=23881
Personally, I wouldn't normally choose this type of place (IMHO - like all chain hotels, they are pretty souless) but for our capital city, this is probably a fairly unbeatable deal. There's range of Travel Inns / Travel Lodges across the country - but go for a family run Tradional Inn or B&B where possible if you want to see real England rather than sales reps.
If you're a gambling man or last minute person (like me!), then if you leave your booking late - try www.londontown.com for a possible 4 or 5 star hotel at a knockdown price.
Try themfor cut price rooms in 4 star hotels in London. Particularly good for filling unfilled beds in the City at short notice.
Also you can download a pdf file listing all the years interesting London events.
I've used this website myself to book hotel accommodation in London, and found it to be very efficient. I booked a room in a 4 star hotel right next to Tower Bridge, for little more than the cost of a cheap B&B up North! It was about £36GBP. A mere pittance for ANY type of accommodation in London. You can normally double this at least. Full room rates for 4-star hotels in London are £200-250 GBP per night. | 
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