>wouldn't the people living above the tubes be happy to get a heat exchanger (passive) or heat pump (active, but takes more of the heat) that prewarms their hot water supply
Ground source heat pumps are expensive to build, even more so in a dense area like London. So even if everything you said is true, I suspect the juice isn't worth the squeeze.
In this case, though, the excess heat is a major burden, so there is room to negotiate with a district thermal provider that pays that provider to absorb and redistribute the heat, as long as it's less than the cost to pump it out to the environment.
I'm not saying it's easy (it will likely be a bespoke solution). Given the organizations, I expect the difficulty to be as much business (setting the prices) and political (defending the prices set) as technical.
Ground source heat pumps are expensive to build, even more so in a dense area like London. So even if everything you said is true, I suspect the juice isn't worth the squeeze.