SafeStor
That's the catchy name for "Remove the spent fuel, make the site safe, demolish ancillary buildings and wait for noticeably radioactive parts of the reactor complex to decay to the point where the rest can be demolished and recycled without contamination issues". I understand SafeStor is supposed to run for eighty years from the beginning of decommissioning, not the end of operations.
Prompt decommissioning supposedly costs about the same amount of money but it's carried out over a shorter timescale and with more chance of radioactive contamination escaping the containment building.
The Calder Hall reactor complex in particular is not going to cost £70 billion to decommission, that (now outdated) figure was for the entire Sellafield site which includes all of the nuclear weapons production waste and residues as well as the waste from the Magnox reprocessing lines on site. Decommissioning the civilian power reactors are not the reason for the pricetag you quoted.
https://www.bbc.co.uk/news/uk-england-cumbria-26124803 (from 2014, linked to from the 2019 BBC News report you quoted)
The Zion PWR in Illinois USA was shut down in 1998, decommissioning started in 2010 and completed in 2023 with the site no longer being licenced as a nuclear facility at the end of the process. This cost about a billion dollars in total. The single Magnox reactor at Tokai in Japan was decommissioned over a similar time period to the Zion PWR operation, no cost given.