Thanks for those links, I didn't realise it was that easy. However, this was in a private taxi and not in London, but yes, I could have reported it.

Honestly though, my problem isn't with the occasional awful ride, but the far more frequent slightly annoying rides. The ones where the driver won't stop talking or asking about local women or good bars in the city or the latest football gossip. I can't complain about that (nor do I really have the right to) but it's a huge reason why I prefer the Uber service.