I have just googled if you can post lithium batteries, this is their answer:
Yes, you can send lithium batteries in the UK post, but there are restrictions. You can only send them if they are contained in a device, and you must take precautions to prevent overheating.
Are you using Royal Mail? There's an advantage here , as they do not charge extra for posting to N.I. whilst other carriers do. Personally then, I would check at a general main Post Office, if there are any restrictions.
@medwards870