Lipos can be sent within the UK via Royal Mail providing they are sent IN or WITH an electrical device.
If it's a hard-case lipo, you could argue that the lipo cells are contained within an electrical device (the hard case), so are safe for posting. I know some shops take this approach.
I had a soft-case delivered by Royal Mail once that came with a single LED, so I guess they could claim it was sent with an electrical device. Probably not within the spirit of the rules, but within the wording, which I thought was a very F1 thing to do
Whichever approach you take, you do need to tell the counter staff that there's a lipo inside, and it needs to be very well packaged, otherwise you'll be in serious trouble if it gets damaged and causes a fire.