I had a friend who was Irish....who flew in the same Battalion....which branch of the Irish I do not recall. My unit supported the Australians down at Nui Dat, and I am aware of Australians being other places as well including the 135th ENU's which was a joint Australian Navy and US Army Assault Helicopter unit flying Huey's.
I have never heard of any British Soldier or Airman being assigned to, attached, to, visiting any combat unit I am aware of.....which does not mean there weren't any....but in all the time that has lapsed....it would seem most likely it would have become known.
I have no doubt there may have been some SAS folks amongst the SpecOps guys but nothing has cropped up in any of the books written about those years by some of the participants that I am aware of.
The Aussies were great to work with.....entirely too organized compared to the American Army when it came ot using Chinooks. Superb Soldiers in my view!