Hi Gill,
The times that I have in the system are;
Gatwick to Iceland depart: 13.25hrs
Iceland to Anchorage depart:17.10hrs
I believe flight upgrades are with the same airline as the main group so flight times would match those above.
As you have already hinted at, these are provisional, subject to change and would be supported in your final documents.
Kindest regards,
Julia