For wheelchair users, simply riding on a double decker bus is an excellent and cheap alternative to a special disabled tour. If you learn the routes and get a day pass, you can ride anywhere.
Additionally, in many taxis, you can just wheel in and go. The underground has many wheelchair-friendly stations and links to airports and the Chunnel, if you want a new and exciting way to explore the continent!
Contact any tourist info booth in London for details, or ask at the stations.
www.bluechairbook.com
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www.tfl.gov.uk/assets/downloads/tube-access-guide-map-0108.pdf
There are only three, yes three stations in central London that have wheelchair access, including all of those on and inside the circle line.