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This is probably policy for no frills airlines.
No. It used to be a sneaky trick to get to the front of the queue that some airlines used, saying they were a bit low on fuel even when they weren’t. Now we have the policy where you declare an emergency, or you get nothing. The implications of declaring an emergency are that reports are generated and an investigation takes place, as @Fox alluded to above.
When an airport like Gatwick shuts down it may be that declaring a fuel emergency is a good way to get to the front of the queue and ensure that you are diverted to Luton and not to Prestwick.