Budget

If you want visit London , you have to think about your budget because London is one of the most expensive cities of the world.
Some travel agencies offers travel packages that include transportation, accomodation and feeding it's a good option if you not want to spend much.
See the links of some travel agencies  and make your budget !

http://www.londonpass.com/prices.asp

http://www.statravel.com

http://www.expedia.com/