Mag,
If the fog had lifted in the morning no doubt you would have expected the flight to go. In order to achieve this the crews at LGW have to be on duty from early in the morning so that they can be contacted and sent to airport at short notice. Even if this was the case it would take around 2 hours from phoning the crew at the hotel to departing. The fact that they are in a hotel and 'well rested' as you put it is incorrect. It is not possible to rest and be on standby for immediate call out at the same time, the only way they can LEGALLY rest is to be off duty i.e. not contactable.
Also how do we know that the whole crew were even available to operate. If they had already had an unscheduled night stop they may well be on a day off the next day, you are then into asking them kindly to work their day off - which I have to admit is not normally a problem with FlyBe crew.
The problem is you only have one set of crew who are away from base and are only capable of doing so much.
Trust me, FlyBe do not cancel flights on a whim.
Look at it from the other side of the coin, the last thing they want is to upset customers.