A less-expensive way to stay in London is to opt for a youth hostel. We've stayed at St. Pancras various time. Don't let the "youth" label discourage you, there are travelers of all ages! For me it has several advantages:
1-While not cheap compared to hostels in other European countries, it is cheaper than a hotel yet still has many amenities.
2 - Includes a very hearty breakfast.
3 - There are rooms of various sizes, with or without bathroom included. If you opt for the shared bathroom, the price drops. The rooms are simple and have bunk beds but they're alright if you just want to get some rest before exploring the city.