Pretty good for a rough estimate!

The best I could find was a document by the National Audit Office from 2017 (using 2016 estimates); page 18, figure 2 - The expected costs of Hinkley Point C.

https://www.nao.org.uk/wp-content/uploads/2017/06/Hinkley-Po...

Back then the construction was estimated at £18.2 billion. Ongoing operation costs (including fuel, staffing, and grid charges) were priced at £29.3 billion and decommissioning (without the uplift) at £7.3 billion. Total project cost was put at £54.8 billion.

Again, it was from 2017 so things have changed.