The British Red Cross loans wheelchairs to people who need them. They are free but the BRC is a charity and asks for a donation from those who can afford it. I used one of their wheelchairs for 6 weeks and gave them something like £50 for the loan.

London area contact details are here:
http://www.redcross.org.uk/localserv...66&cachefixer=

Another site that I have bookmarked but didn't use is:
http://www.fredwalton.co.uk