The remedy slots used by Flybe are specifically linked to destinations served by both BA and BMI at the time that BA acquired them. There were no US destinations in this list.
If JetBlue want to fly to LHR (or probably LGW too) then they'll need to open the cheque book and pay for it like anyone else. I see no reason for them to get preferential access to slots when these would not be afforded to any other operator wanting to access the same routes.