Is TfL not monetising this thermal mass? Why not?
Heating and cooling benefits from remarkably small differences in temp. Thats how heat pumps work. And, lifting the input temp by only 5-10 degrees C can make a huge difference to the energy required to get to the next level.
It takes about 200Kj to heat 1L of water from 10C to 60C and 125Kj to heat it from 30C to 60C -A 37.5% saving in energy inputs. You better believe a corporate would seek a 1/3 reduction in energy input costs.
(my sarcasm meter is broken sorry if I ruined a joke)
The line is itself a giant distribution system and is pretty much designed to retrofit pipes into.
The commercial premises would merely have to tap into something which is within a few tens of metres of their basements, and in some cases they very probably have access hatches already.
You are heating your working fluid to a temperature equal to what's ambient within the heat exchanger, which takes the longest possible time of any heating this exchanger can perform. (Think it over.)
For the exchanger to operate continuously - that is, to sustain outflow at final temperature equal to inflow at initial - this means you need a lot of piping, because however many CFM of water at 10° (or whatever) come in, you need to supply exactly that many CFM at 30° out.
All that piping takes volume, which has to be excavated out of the clay in which has been sunk the heat we're striving to remove. There's no moving that without some heat exchanger, after all. So we're still stuck expressing time basically as volume, which is perhaps the worst possible misfortune when working underground. (Caissons can be built and pumped dry, where earth must be dug.)
Perhaps you're thinking of a cooling jacket lining the tunnel bore. This would make sense but will only operate at a net energy cost, because to sustain livability you're going to need to exchange heat outside the system and supply, effectively, refrigerant. From plants all over town...