The comprehensive A-Z list below gives you detailed information about the things you cannot post with us, or where restrictions are in place and covers all our UK mail and International mail services.
https://personal.help.royalmail.com/app/answers/detail/a_id/96
Electronic devices sent with lithium batteries (including mobile phones, digital cameras, etc).
Not allowed for international postage. Allowed for UK postage with restrictions. Restrictions are like 30 lines long.
For lithium metal/alloy batteries the lithium content must not be more than 1g per cell or 2g per battery. The maximum net quantity of cells or batteries is 5kg per parcel. 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. Cells or batteries that are defective or damaged are forbidden. 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. Must be presented at the counter. Sender’s name and address must be visible on the parcel.
I don't know how much lithium is in a battery tbh but 2g per battery and 1g per cell sounds low to me.