I did the same thing with Carvana. The experience was wonderful until after I got the car. They waxed the stupid thing with the windows down. There was wax in random places EVERYWHERE in the car. I asked for money for an internal detailing (I was willing to just deal with the external part having the worst buff job I've ever seen). They offered $75 and the lady on the phone said she could get her SUV detailed for that much. In Texas.

Anyways, in the end, you can get them to do almost anything by simply saying you want to return the car. That's pretty damn expensive and something they have no choice but to honor. Still, if you don't know this trick, it's a good way for these guys to be sneaky. This would've been easier for me to deal with if I could just physically bring it in somewhere and say "give it back clean".

That being said, I also understand you have better consumer protections in the UK, so maybe things are different for you.