After defuelling, the Nuclear Decommissioning Authority (NDA) is responsible for decommissioning in the UK. Decommissioning is carried out using one of these three methods defined by the IAEA:
The station is defuelled, dismantled and decontaminated as soon as possible after shutdown. The site is quickly cleared and made available for reuse.
Any high-level waste is removed and most of the installations are dismantled except for the reactor itself, which is put in a safe state. The defueled and safe power station is monitored for 40-80 years while its radioactivity levels decay. Then the final deconstruction of the reactor can be completed.
All radioactive material is placed in one part of the power station, which is then encased in thick concrete. This option is being pursued in some parts of the world, but it isn't actively promoted within the international nuclear community.