Sellafield isn't an ordinary nuclear power station. It was part of the UK's nuclear weapons program and contained some of the UK's earliest (maybe the earliest) nuclear reactors.
It's not going to be representative of the decommissioning costs of modern generation II or generation III reactors.
Regardless of how much one is for or against nuclear power, i think we can all come together and agree strongly that building an air cooled nuclear reactor that uses a flammable substance as a moderator and blows exhaust out a chimney is a catastrophically bad idea.
And I don't imagine that kind of activity results in easy cleanup.
Also, AGR fuel needs to be condensed or reprocessed shortly after discharge. It's great in a gas reactor, not so much in a spent fuel pool. That means much of the UK's discharged fuel as been consolidated at Sellafield. But all that plutonium is a great fuel resource. Jealous of whoever gets to fuel their reactor with that goldmine.
Sellafield is a large site consisting of many facilities [1]. These include Magnox reactors [2] which were historically used to create weapons grade Plutonium and are much "messier" than typical commercial power reactors. The Sellafield site has been and is still home to a very large spent fuel reprocessing plant that handles spent fuel from numerous UK reactors. Reprocessing plants are an entirely different beast. And a lot of the activity at the Sellafield site is generally from older, messier technologies as well.
https://en.wikipedia.org/wiki/Nuclear_Decommissioning_Author...
Current estimates for one Nuclear Power Plant Site (Sellafield): £114bn which seems A LOT!