With regards to the comments over heat, there is a large difference in how useful large amounts of low-level heat (e.g. just above room temperature) and the sort of heat levels used to heat water, or generate steam for electricity generation.
With regards to using the heat from tube tunnels to warm local buildings, I can assure you it's not something that has passed the Cooling the Tube team by. Unfortunately there are a number of fairly serious reasons against it. Such as running however many miles of pipework about from tunnels underground, with buildings covering them, to somewhere useful. Then there's also the heating systems used in modern buildings, I believe they are more distributed now rather than a single boiler room in a basement, which means the complexity of piping the heat about the buildings also becomes massively more complicated and expensive. Heat re-use solutions simply haven't stood up to the analysis, being impractical or far too expensive.
Saving £2 a week on the radiator bills doesn't justify the hundreds of millions (if not more) widespread adoption of such a scheme would cost to implement.
London Underground are serious about climate change, as well as making the temperature of the network more bearable in summer. The Cooling the Tube team have been investing serious time and money assessing real-world solutions, unfortunately so many of the 'ideal' suggestions simply collapse upon scratching the surface.
One of the best things being done to cool the tube is by reducing the amount heat being generated, the traction system (friction braking and traction rail losses) is a major heat-source (passenger body heat is fairly negligible by comparison) - so they're looking at reducing this heat generation by increasing the use of regenerative braking and lower-loss conductor rails. Better to stop making the heat in the first place than then have to try and get rid of it.