If it was prescribed for fear of flying, that is a use against guidance and therefore not a defensible position for the GP.
How many cases have there been of GPs being sued for the consequences of taking 2mg of a benzodiazepine or some similar low dose of a generally well-tolerated sedative drug, losing the case, and the MDU or equivalent taking the nuclear option and refusing to pay on the grounds of the precise details of the basis for the prescription?
I mean, given the fear of this that GPs are apparently living under, it must have happened at least once.
Because I call bullshit. I don't believe such cases have happened. I don't believe "I took 2mg of a sedative and tripped on the way onto the plane" would result in a successful court case (because there's nothing special about plane steps which isn't true of any other steps, and plenty of people are taking prescription pain killers and sedatives without living in bungalows) and I absolutely don't believe that the MDU would attempt to get out of payment. Every day there are many, many people travelling on planes under the influence of drugs of a similar strength, because they are standard prescription drugs.
It's not off-label prescribing, the CAA aren't a statutory body for the purposes of drug usage and a search of the MDU website for every imaginable keyword shows no cases, "cautionary tales" or guidance. I think this is GPs making up things to worry about.
I get that it may not be effective medication, or may have undesirable side-effects, and that's a medical judgement doctors are perfectly entitled to make (although the propensity of GPs to engage in n=1 anecdote means some evidence would be nice). But "I'd otherwise prescribe you that but won't because I risk being sued and the MDU refusing to defend me?" Nah.