Prat is another word for arse. Which, by the way, there is no such word. The word is ‘ass’ but Brits stick an ‘r’ in there so it sounds like it’s spelled differently. If they spelled it ‘arse’ they would skip the ‘r’ and it would sound like our version of the word.
kinda like a Cocknied “warsh” for wash?
I use ‘arse’ quite often, mostly in exchange for the word “faggot”.