Where is the cheapest place in the UK for genuine batteries?

Which drone?   

If you don't mind copies or OEM batteries then Amazon is probably the best all round choice.

For the Phantom 3 series: Powerextra, EBL or Mackertop are priced around £58 I've bought the Powerextra batteries and had no issues with them at all.
Some others on there listed at £116 I think for Phantom 2 and Vision etc, Phantom 3 £122, Inspire 1 4500mAh £154 and Inspire 1 5700mAh £191.

For Phantom 4 batties the cheapest are about £80 (again Powerextra) upto £172 not sure if that top end price is for a geniune one though.
The only way you'd know they're not geniune is the power level LEDs are a different shade of green.

eBay may have some cheaper stuff but it is also an ocean of sharks, I'm trying to get my money back on a fake RC unit. I'm not saying I completely trust Amazon but there doesn't seem to be any decent RC shops in the UK anymore, not locally anyway.
Anyway hope this helps

It is recommended to purchase from local authorized dealers: https://www.dji.com/where-to-buy

Drones Direct, or check out Maplin if they still have any left, unlikely though