Why does privatisation mean that the government can't build infrastructure? I think the answer is more likely nimbyism than privatisation.
I personally think it makes no sense to privatise infrastructure for which no competition can reasonably take place, and I'd include distribution networks of many sorts in that: water distribution, rail lines, electricity distribution.
But I'm not aware that privatisation means that the state can't take on reservoir projects. The problem is that development of all kinds in the UK is utterly crippled by nimbyism. The article mentions the proposed Abingdon reservoir but links it to the boogyman of data centres rather than call out what the picture obviously shows: it's being stalled by nimbys.