Mag,
I wish it was that simple ! Unfortunately you can't just keep crews waiting in a hotel ready to scramble at short notice. I am willing to bet that the crews had been on duty from 06.00 am having done the red-eye JER-LGW and that they had remained on duty at the airport in the hope that the weather would clear. From that time in the morning you are limited to do about 11 hours so unless they were sure of landing at JER before 5 pm then they had no choice. In order to land in JER before 5 pm they would have to start the ball rolling in LGW about 3.30 pm, i.e. - 1 crystal ball needed.
As Airlines tend not to have fresh crews on standby ready to operate at short notice living near all the airports they operate into (they tend to be where the A/C is based, you find things run much smoother that way
) then the only option is to cancel so that you can get your only and knackered crews to the hotel ASAP so that they have the minimum required 11 hours rest so they are able to have another go first thing next morning.
If the fog had cleared a couple of hours earlier then things would have been different as FlyBe tend not to cancel flights UNLESS IT IS ABSOLUTELY NECESSARY, unlike some others I could mention.