It's due to fire hazard. If it's Dell, then there's a genuine concern, but other than that, I don't think there have been many fires on non-Dell laptops due to rubbish batteries.
Yodel state (at https://www.yodel.co.uk/sending/get-more-detail/prohibited-restricted-items.aspx):
... Dangerous Goods (these include but are not limited to lithium batteries,
So I suggest you ring, and ask if they can ship a complete MacBook, mobile phone, iPod, digital camera... they'll say "yes, of course", then state that you need to send the battery, tomorrow as the other bits are in transit at the moment and you're waiting for them to arrive. It's a bit hard for them to go back after that. You then have agreement to arrange collection and delivery. You could avoid the white-lie and just say "don't say you can't ship a battery then".