Most of the Hinkley Point C and Sizewell C costs come from interests being paid in the high risk debt raised to build it.

Unsurprisingly it turns out financing such a long term project on a 10% rate debt is not a good idea. (Keep in mind that this debt has been borrowed during ZIRP, where the baseline interest rates where near 0, this rate is just the premium that private actors are taking for the risk to see such a project being canceled for political reason). Of course the sensible way to do it would have been to finance it on long term public debt guaranteed by the British govt, who would then have born the burden of an hypothetical cancelation of the projects (and the cost per MWh would have been 4 times lower IIRC), but public is a forbidden word for the Tories.