Havana I'm aware of the insurance requirement for planes, their components, servicing, crews, airports etc.
I'm just not sure whether some insurers might take a couple of weeks after a no-deal / inadequate deal Brexit to realise all the relevant implications:
much of UK business seems oblivious and imo even some EU businesses haven't fully understood all the possible ramifications.
I expect even the EU notices to stakeholders will have omitted some details - it is an incredible undertaking to cover all the problems that may arise when the world's No. 5 economy and No. 2 financial centre are taken off line.
We have no precedent in modern times
Military airlifts are of course the emergency fallback and the RAF should have sufficient lift capacity for essential meds & isotopes,
but not to bring in a significant % of current food imports for the nation.
We might need to ask the EU to help with their military, if food supplies were really held up,
but I really don't expect that to happen.
Maybe some local shortages, especially in rural or very poor areas, where the military might need to distribute existing supplies.
More likely is the emergency WTO waiver that lets in every lorry & container in without checking, including not just dodgy food but drugs, weapons, even slaves.
Remember the Chinese baby formula scandal of some years ago, with thousands hospitalised and many deaths.
We don't know what could get into the shops or food chain.
Maybe even show up only years later, in a chain of events, e.g. as unsuitable animal feed led to human CJD.