Have you seen the article in January 2018 RadCom, page 74?
There are all sorts of new regulations about sending batteries through the post in the UK. The rules are even more complex than the rules about taking batteries on aircraft!
It seems you are not allowed to put ANY kind of used batteries in the post - whether or not they are installed in equipment. This would seem to preclude sending mobile phones, iPads, etc, which have non-removable batteries if these have ever been used.
There are especially restrictive and complicated rules about sending Lithium batteries in the post. It is alleged that there have been cases where packages have been seized and destroyed by Royal Mail with neither warning nor compensation.
A different set of rules applies to businesses which have an agreed contract with Royal Mail.