Why 'definitely not traditional British folk'? Replete with chances for harmonies and co-operation. I should begin with the Copper Family — 'Thousands or More' would be a good one to start with - and work out from there. Then try both well-known tunes of 'Byker Hill'. Then 'Holmforth Anthem'. Traditional British folk offers infinite possibilities. Any shanty will give you plenty of call & response opportunities.

Enjoy!