I remember listening in the staff room to somebody who said: ‘And there they were, all sat at their desks ready for the lesson.’ This sounded quite foreign to me and not something I would ever have said. My version would have replaced ‘sat’ with ‘sitting.’ ‘ He was sat at the front,’ sounds even worse.
I tried to work out what my problem was! I do of course use the word ‘sat’ as in ‘he sat at the back and I sat at the front’ but for me ‘sitting’ works far better unless the action is truly in the past. Sitting is I suppose the present participle of the verb ‘to sit’ and the present continuous tense. Thus ‘I was sitting’ is the past continuous tense and various sites online all declare that ‘I was sat’ is quite simply wrong!