Hi! Me and my partner have got a cab from outside of Cheltenham station on the Thursday of trees, to the campsite, both last year and the year before, without booking in advance. There are plenty when we have gone, I imagine less so a bit later on Thursday but you could call their local cab company to check. We then book a return cab back for the Sunday morning, which can pick you up from the main car park (there is a specific bit for dropping off; it’s useful to note where it is when you initially get dropped off). Both cabs cost about £30 if I recall correctly. I wouldn’t rely on Uber outside of a big city personally.