Hanford could be handled for far less (no idea about the UK's sites but probably the same situation). Its the result of making the atomic bombs during WWII. The acids could be neutralized and the uranium separated out. However, a contract that just keeps putting the material in new containers every few years is much more profitable. The idea it should take hundreds of billions to do is pretty much about an nearly infinite time horizon of maintenance instead of just cleaning up the site.

Also, Hanford has nothing to do with civilian NPPs and the processes used there would never be done as part of power generation.

Finally, the cost of NPPs is almost always about the lawsuits. Look at how much Korea and China payed for their NPPs. It was less than 20% of what we pay, they were online in less than 5 years and they are producing 2.8GW for each country for the last decade.

PS The UK has never deployed a standardized design for a NPP. All of their NPPs are different types of experimental designs. Just buy a AP1400 and be done with it for far cheaper.