but then commuters don't expect mobile phone reception in any another
below ground station

I find this very astonishing. In other parts of the world mobile phone
coverage of underground lines has been custom for years. The operators
have installed several transmitters (e.g. here
http://www.fst-gsm.de/d1-u-bahn.html is a list for one operator in
Cologne, Germany. The other 3 German operators have similar equipment).
It works.

As far as I know the phone operators pays the transport company (KVB) a
rent - so KVB gets more revenue and improves service at the same time.

Of course, the London underground network is much, much larger. But even
then I find it hard to understand why it is that difficult there.