This news article suggests that the newer London taxis can take electric wheelchairs, but in practice actually getting a taxi when you're in a wheelchair can be problematic:
http://news.bbc.co.uk/2/hi/uk_news/magazine/7294746.stm

(The relevant quote about the electric wheelchairs: "As Ms Bashall uses a large electric wheelchair, we're on the look-out for one of the newer, wider TX cabs." 4th paragraph.)

The suitability of the TX4 for electric wheelchairs is confirmed here: http://www.worldcarfans.com/10610195939/new-british-built-tx4-taxi-world-launch