It's true that all else being equal new power demand lowers the fixed cost per unit of marginal energy generated. But if the data centre requires new grid and storage capacity to handle its load and those fixed costs are charged to the entire electricity consumer base, there is a trade-off.

Also, all else is not equal because the UK is already grasping towards a stretch target of a >95% low-carbon power system by 2030 which has over-loaded supply chains, sacrificed generation cost to speed, and created a queue to connect to the grid that outpaces the annual rate of connections by over an order of magnitude. Adding huge blocks of new incremental demand to the system is quite a challenge, in this context.