Probably a 3008 or an Arona (Groups D and E). If you're lucky, perhaps a Golf/Audi A3, or even an A-class?
I'd suggest asking at the counter upon collection - nothing lost if you're interested in something else and they say no.
I appreciate the response. Yuck. Definitely don't want an SUV. Does Avis in the UK give a selection in the app prior to pickup? It did for me in the U.S. the night prior, although I ended up sticking with the car they eventually assigned to me.
In 2019 I ended up picking up a Golf GTD wagon for the price of an economy car from Stansted (no status at that time, just a lot of excitement upon seeing one and the manager was all too happy to oblige). Ah, good times...