From the Royal mail website.
Batteries - new alkaline metal, nickel metal hydride (NiMH) or nickel cadmium (NiCd)
Including D, C, 9V, AA, AAA and AAAA alkaline batteries. Must be new and sent unopened in their original retail packaging. Surround with cushioning material e.g. bubble wrap. The sender’s name and return address must be clearly visible on the outer packaging.
Batteries - lithium ion/polymer batteries sent with, but not connected to, an electronic device The maximum number of batteries allowed in each package is the number that may be connected to the equipment plus two spares. 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 packed in inner packagings that completely enclose the cell or battery. Cells and batteries must be protected against short circuit, including protection against contact with conductive materials within the same packaging that could lead to a short circuit. The equipment sent with 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.