It's a messy subject these days, Lunacie. Both in UK and US there has been a lot of drift and a lot of "new" wicca forms [ Faerie wicca, Norse wicca, etc ].
I would hate to tell you the battles I have had over this over which wicca is "real" and which is not.
Much of the difference between UK and US is down to culture and heritage.
Wicca is at least partly based on the old Traditional Witchcraft of Britain which goes back to pre-christian times as a pagan nation [ still is ! ] whereas the US was founded as a christian country so there is no depth of pagan heritage. Heritage/History has a major bearing. Give you an example. I live in a city that was ancient when christ was born -Bronze Age and Iron Age remains with some suggestions of even earlier Neolithic building. I walk a lot in my city up to the Norman Cathedral which was built over a pagan holy site, along Roman roads. Beltane, we had our festival in a henge close by which was built around 5000 BC and we invoked a pagan goddess who was worshipped by the Celtic tribes. There are places that I walk and "feel" all those ancestors - it soaks into the soul and spirit.
So, put us in the UK as Old Wicca [ I do not claim to be wicca but something a lot older ] and maybe the US as new Wicca. As I said, Solitary is the norm although Working Groups are relatively common also - not covens but groups which come together for "special projects" and socials.
The forum where I am a mod. has [as far as I know ] no members of covens but has several HPS's who gave up covens. Hope that answers your question
[ p.s. meant to say there are some forum members who predate Gardner - I hang out with old witches
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