10-11 Sizewell C's, assuming the UK 2025 average demand of ~33 GW.

A couple more to deal with outages. Plus some battery capacity, which you want because nuclear doesn't load follow well. At £38 billion per, you need about £500 billion to power the whole country. About 40% of one annual UK budget.

It's possible to imagine building Sizewell C's at considerably lower cost, as is done it other nations that exist, and manage to build such things without the huge multipliers, but that's another matter.