I have a 'leg' of the family from South Yorkshire and as well as snap, I've heard 'jock' used.
I think the miners used to take a jock tin darn pit !
The term 'pack up' is always used by Mrs I. She is from a more urban area of West Yorkshire (
) and it is purely a term for snap and not a full blown picnic.
On the other hand I'm from very West Yorkshire (i.e. not central West Yorkshire - there is a difference): Calderdale - and we were so poor, we didn't have any snap/jock/shim shams/pack up/luncheon/bait or piece
Luxury !