The majority of the Gatwick 777 fleet does not have a first cabin. As soon as there is an issue with a four class bird there are few options; (The 787 engine issue and A350 crew shortage mean that there is little scope for a 777 from Heathrow to cover.)
No easy solution to this. I would book F and claim the 75% refund if it happened. ! Few other options from Bermuda.