From what I know (as an employee at NEXT - Westfield London) The 1 day annual sale starts at 5AM on the 27th of December.
When I asked my manager what will be on sale exactly, she said its more or less everything but there will be the odd few things out that wont be on sale.
Anyway, this is kind of early, but I thought I would give you guys a headstart on Info, and plus - these sales are always crazy and people always queue up hours before so you better be in early if you want to grab the good stuff!
Oh and by the way - this applys to all NEXT stores (in london as far as im concerned), not just Wesfield.
THE SALE WILL INCLUDE THINGS SUCH AS:
Womens Shoes / Slippers
Womens Night wear / Pyjamas
Same for mens too.
Kids Wear & Home wear
I will update this as I learn more.