Hi OP. Ask your normal GP for a private prescription as another poster was advising.
Folk cutting and pasting the possible effects of drowsiness, circulation, etc. FFS, you'll be given 2mg which will - as you stay - stave off a panic attack, but do little else. People drink alcohol on planes. That should be BANNED then before flights and on them because the same effects - drowsiness, inability to act in an emergency, messing with your circulation, etc... your Gin and Tonic is going to do that to you.
Benzos understandably have a terrible rep. because of addiction issues, and I think you're unfortunately tapping into some "all drugs bad" reactions here.
I have a friend who does a lot of international travel for work and takes diazepam for flights. Like you OP, she has a terrible phobia of flying which she's never been able to overcome. Like you, she's tried a fear of flying course. She's tried hypnotherapy and nothing works. So she takes 2mg of Diazepam for a flight and has done so now for 40 years without problems or issues. Her GP prescribes her a few at a time she says.
What's ridiculous about stopping you from taking the drug that works for you here, is that a person might end up taking an antihistamine, a bunch of Kalms tablets, washed down with a glass of wine to cope with the flight. It's just stupid, when you are a responsible adult who knows that an extremely low dose of a drug you've taken before works for you.