Pork pies need English mustard?

No, but they taste better with SOME mustard. My preferred choice is wholegrain mustard.
As for snags (sausages/bangers), Richmond do thick, thin, and skinless. All of which are OK.
Never heard of the Bolton Snag Co, but I'm willing to try anything in order to find a good one.
Problem here is that most sausages are dry or plastic-looking, just rubbish.