I feel for you! I was in a similar position a few years ago.
Have you had anything like this before / do you have any idea why it happened?
Do you know where you are going - if you can have a nice time without needing to walk too much?
If you're familiar with something like this, think it will be short-term,and you can handle it, then gamble on it getting better, take your medication (and yes, I think a pharmacist's advice would be useful) and go. Ring the airport for help (they are used to this). But do consider what insurance cover you have(see below)
However, if you have any inkling that it is unfamiliar or might worsen then cancel, and claim on your insurance.
If you do that, contact your GP first thing on Monday, explain to reception that you don't need an emergency appointment (assuming you don't!) but you do need to log this and request a letter for insurance.
If the insurance company can't be contacted at some times, then they will expect this. I was able to email mine so it was logged.
Georgesgran - I think this might have been good advice prior to Brexit. Now Betty will need to check exactly what is covered on reciprocal care - and - as she has already seen an osteopath, it may be considered a 'pre-existing condition'. This means that reciprocal care (and possibly travel insurance) might be invalid. Betty, I would check very carefully on these.
My own insurance company were excellent - the GP gave me a letter based on history taken over the phone - and the insurance company repaid me in full. So I was out only the cost of the GP's letter.
I know that although I was very disappointed, I was able to get care, heal properly, and enjoy a holiday later. I hope it works out for you, whatever you decide.