My Burberry SA said it was a private sale only. And it seems to be up to the discretion of the SA who gets to know.
When my husband and I were there and my SA told us, we were led to closet of the the private room in the back and we were literally the only people there.

(Pre-pandemic it was not like this, I definitely recall at least a handful of people at the sale and it later went public 1-2 weeks later)
The selection was limited (especially in sizing) but like
@Christofle said, ~65% off most items (I think bags were around 50% off in my location) My husband got a bomber jacket and I got a tuxedo blazer and wow, it was an incredible discount!
Edit: oh sorry, about the classic trench coats, no, none of those were on sale. But there were definitely some very classic-looking trench coats that were very nice. If I lived in a colder climate I would have definitely considered getting one haha.