Their fate was not sealed on leaving Exeter, as after leaving Exeter they must have passed/been within range of airfields that were open and with better conditions than Blackpool.
Knowing they were not flush with fuel, at 45 mins from Blackpool, for example, they could have a) checked Blackpool's actuals (by phone if needed), re-checked their fuel state and decided to divert, as they would have known they did not have enough fuel to attempt Blackpool and then divert.
Basic en-route checks of fuel, distance to run and actuals at the destination would have given them loads of options.