I have been making plans to travel to Europe for some time. One of the websites I frequent is http://www.eurocheapo.com/london/ I think you will also find information for more than just London.
I also recommend you check out http://www.travelnotes.org/ for more information concerning your travels.
I have been told by several European friends that staying in B&Bs is quite the experience in all of Europe. I'm sure with a little internet snooping you could probably find a few that appeal to you.