Dropping someone off at a UK airport usually costs around £6.51 on average, but...

https://dlf-ne.org/is-paying-two-barrier-fees-ever-the-right-move-at-uk-airports/

Dropping someone off at a UK airport usually costs around £6.51 on average, but it’s £10 at Gatwick and £7 at Heathrow. We'll break down how these fees compare to parking costs, plus what pick-up charges look like