Are people on the Honored Preview Event in London useing the Seeker/Keeper roles picked at gen-con or can they choose freely ?

There's nothing I can find that says one way or another. All it says is only one core will be distributed. I would guess that means you'll be able to pick freely, but stay tuned for updates.

In the Smokeless Fire article it states the deck building rules and suggested deck list presented are for use in both of the GenCon tournements and the honored preview event.

So unless they announce a change after Fridays Top of Clan picks we should be able to choose which role we use in London.