That
sounds shocking - but there's no way to tell if it is actually shocking if you don't have some data to indicate what level of leaks is reasonable given what today's technology can achieve, and the state of the infrastructure at the time that Thames Water took it over.
Since I don't have access to any of that kind of data, for my part, I simply have no idea whether or not that level of leakage is reasonable. (Although as a London Thames Water customer I certainly won't be happy if water rationing comes in)
Is it insane? Do you have any data on how much it would cost to improve the infrastructure to significantly reduce that level of leakage, as compared to the cost of accepting that leakage? Without knowing that, it's impossible to know how (un)reasonable that level of leakage is.
Ah yes, of course, I'd forgotten, how nationalization is the universal panacea that miraculously fixes all problems, without even any need to figure out what was causing the problems in the first place.