Competition among ride services should keep this kind of thing in check. If you're in an area where Uber is the only option (don't know how likely that is) you may be stuck.
There are competitors to Uber in London [2], myTaxi seems to be the one that lets you hail London Cabs via an app. And apparently they're offering 33% discount over Easter [1]. (Never used them and I'm usually an Uber fan, but it seems myTaxi could be an alternative to try.)
[1] https://uk.mytaxi.com/EasterDiscount
[2] https://www.pocket-lint.com/apps/news/uber/142333-best-taxi-...
'Standard' taxis are really expensive, I would say 2-4x the price of Uber, so a 33% discount doesn't really get you anywhere.