Although you are right they can be posted, Royal Mail T&Cs clearly don't allow single batteries to be posted. Their prohibited goods T&Cs clearly say 'Lithium ion/polymer/metal/alloy batteries when not sent with, or contained in/connected to an electronic device, are prohibited'. Whether it's legal issue or not Royal Mail can decide what they take (their loss of business), the 'jobs-worth' is only complying with RM procedures that they have to, if they don't then is their head for the chop.
With companies like Royal Mail with hundreds maybe thousands of customer parcel receiving post offices throughout the UK it's easier and cheaper for them to decide on a blanket ban rather than train and monitor so many different post offices to ensure compliance with the rules. If they had only a few post offices it may be easier the train people and ensure compliance with the rules. Sometimes it not about whether it's legal or not but more to do with ensuring compliance with the rules.
I've find the info below on Royal Mails website:
Restricted goods - personal customers
Things we can carry in UK post but with restrictions
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[emoji768] counter.
Lithium ion/polymer batteries sent in isolation are prohibited. Please see
www.royalmail.com/prohibitedgoods
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 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. The consignment must be accompanied with a document with an indication that the package contains lithium ion cells or batteries; that the package must be handled with care and that a flammability hazard exists if the package is damaged; that special procedures must be followed in the event the package is damaged, to include inspection and repacking if necessary; and also a telephone number for additional information.
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.
Lithium battery handling label to be applied (
see an example lithium battery label ). The sender’s name and return address must be clearly visible on the outer packaging.
These items must be presented at a Post Office[emoji768] counter.
Lithium ion/polymer batteries sent in isolation are prohibited. Please see
www.royalmail.com/prohibitedgoods
Prohibited goods - personal customers
Items you cannot send in the post
Batteries
Batteries that are classed as dangerous goods by the latest edition of the International Civil Aviation Organization’s (ICAO) Technical Instructions are prohibited. This includes wet spillable lead acid/lead alkaline batteries (such as car batteries), used alkaline metal, nickel metal hydride (NiMH), nickel cadmium (NiCd), zinc-air batteries, and damaged batteries of any type.
Lithium ion/polymer/metal/alloy batteries when not sent with, or contained in/connected to an electronic device, are prohibited.
Lithium ion/polymer/metal/alloy batteries are allowed when sent with or contained in/connected to an electronic device, but are subject to packaging, volume and quantity restrictions. Please see
www.royalmail.com/restrictedgoods.
For more information on lithium batteries please see the
IATA Lithium Battery Guidance Document .
Sealed lead acid batteries are allowed in the UK, but are also subject to packaging, volume, quantity and labelling restrictions. Please see
www.royalmail.com/restrictedgoods
Alkaline metal, nickel metal hydride (NiMH) and nickel cadmium (NiCd) batteries may only be sent when new and in their original packaging. Please see
www.royalmail.com/restrictedgoods
UPS seem to have a good guide that explains what they can take (link below):
http://www.ups.com/media/news/en/ca/intl_lithium_battery_regulations.pdf
It looks like UPS may be worth trying.