[stumpiey]

a simple search on royal mail's web page yields many results

Batteries - lithium ion/polymer batteries contained in/connected to an electronic device
Each package must contain no more than four cells or two batteries installed in equipment. The maximum net quantity of cells or batteries is 5kg per package. Watt-hour rating must not exceed 20Wh per cell or 100Wh per battery. Each cell and battery must be of a type proven to meet the requirements of each test in the UN Manual of Tests and Criteria, Part III, section 38.3. Batteries are subject to these tests irrespective of whether the cells of which they are composed have been so tested.

Cells and batteries must be manufactured under a quality management programme as specified in the International Civil Aviation Organization’s Technical Instructions for the Safe Transport of Dangerous Goods by Air. Cells or batteries that are defective for safety reasons, or that have been damaged, are forbidden. Any person preparing or offering cells or batteries with or in equipment for transport must receive adequate instruction on the requirements commensurate with their responsibilities. Cells and batteries must be protected against short circuit.

The equipment containing cells or batteries must be packed in strong rigid packaging and must be secured against movement within the outer packaging and packed to prevent accidental activation. The sender’s name and return address must be clearly visible on the outer packaging.
These items must be presented at a Post Office® counter.

Lithium ion/polymer batteries sent in isolation are prohibited.
i.e if it's in a phone or something it's ok, but cells on their own are prohibited, courier time