A patent (once granted) is a piece of property. It is yours to sell as you wish.
Once you have agreed a price with the purchaser of your patent you need to get certain documents in order to suitable reflect and register the transaction.
1) Patent Assignment
Any sale of a patent should be reflected in an assignment agreement. This will (amongst other things) hold details of the patent, those who are party to the agreement and the amount that is to be paid for the patent. This should then be signed by both parties.
2) Form 21
The UK Intellectual Property Office will need to see a copy of the assignment above. This will need to be accompanied by a completed and signed Form 21. This should be sent to the UK Intellectual Propetry Office within 6 months of the transaction.
Once these two documents are filed and acknowledged the transaction is complete.