You could try checking out the travelodge.co.uk and premierinn.co.uk chains as they should be starting their autumn promotions by now. Hotel prices drop quickly the further outside Central London you are and provided you choose a hotel close to an underground station getting into Central London is easy and inexpensive ( search for Oyster card ). Check out lastminute.com as they often have attractive theatre deals depending upon what you want to see.
You can check out the London Pass ( londonpass.com ) to see if it covers what you want to see and if it is better value than paying for things individually.