Had a look at the Avoncroft website - couldn't see anything upcoming about wigwams and tepees - there's an American Civil War thing on 20th September - not quite the same!
As to Austin 12s my friend Bob tells me that there are no major gatherings planned but the Austin Register do have lunchtime meetings at at the Fleur-de-Lys pub on the canal at Lowsonford, Warks on the 2nd Tuesday of the month - their website suggests contacting member Barry Davis - see
https://www.vintage-austin.co.uk/maps/lowsonford.html - Barry will probably know how many 12s are expected in the pub car park and available for viewing on the day.