Metropol

According to London Underground themselves:

http://tube.tfl.gov.uk/content/faq/tipsac.asp

"True air conditioning poses real, but not insurmountable, difficulties on an underground system, because of the problem of dissipating the heat produced. Tunnels can become dangerously over-heated and the temperature on platforms is likely to soar.

Only the most recent metro systems, those of Hong Kong and Singapore, have air conditioning on their trains and, even there, they are experiencing great difficulties in making it work. "

Also as it says, there is not enough space on a lot of the network for AC units due to the small size of the tunnels.

I saw a report on ITV News Channel earlier from London Tonight saying that some buses in the Capital will be experimenting with Air conditioning, but this will be only on a few services, which in the Capital is literally only like 2 or something low.

I haven't got on a Bus for ages now, but if it isn't neccessary I tend to walk.

Aston wrote:
However, in London the underground lines are just that - deep underground. There's nowhere for the hot air to go and thus air conditioning systems just wouldn't work...

Well it's just the oldest lines that don't have any air con systems. Happily a system has been devised to solve the problem on these ones too now! What's been put forward is that the groundwater (which is nice and cold) is pumped up into the tunnels and the hot air blown across them. That of course cools the air the the heated water can be taken away to somewhere useful. Clever stuff if it works, very resourceful.