They posted this on the website, regarding the Dreamliner flights to London during the September-October period:
"As of September 2017, EL AL Dreamliner aircraft will operate to European destinations. During this period, seating in Premium Economy will be offered with Economy Class service. The price for the seat is $200 or 600 points per direction.
Premium members from Gold tier and higher, and those with S, M or Y Class tickets are entitled to Premium Economy seating at no cost"