Source: Gary Leff - View from the Wing
For 2 weeks the mileage cost of economy redemptions originating in London to 20 cities has been cut in half. Travel is permitted October 1 through February 28, excluding December 10 through January 10.
Here are the destinations:
A US resident could presumably book their outbound at full price (in a premium cabin even, or using a different mileage program) and a one-way return using this sale. Folks originating in the UK have it much better in terms of taking advantage of this sale.