Found this out today; a recent free addition to their service.
http://www.three.co.uk/Support/Free_WiFi_on_London_Underground
Excellent stuff.
http://www.three.co.uk/Support/Free_WiFi_on_London_Underground
Excellent stuff.

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It's good stuff, I've used it a bunch of times while waiting at stations but on the tube itself it varies (at a very obvious guess due to it acting as a sort of faraday cage).