sleeping mat recs?

I’m going to be camping for 2 weeks and don’t want to end up not being able to move after the second night. I have no idea about mats and what makes a good/bad one so any recommendations/ advice is greatly appreciated.
My budget is ideally under £50, I don’t mind buying second hand if it’s more pricey new, and heat/ cold isn’t an issue in my situation.
I don’t have to worry too much about the size or weight, I’m driving there but ideally it wouldn’t be HUGE.
I’m not sure if this is relevant or not but I can not sleep on an air mattress, I can’t handle waking up at 4am swamped by a deflated mat of doom.
TIA!