//If you go abroad to somewhere that requires a test or proof of vaccination would you stand there at the airport and argue your point.//
I'm not talking about abroad. I'm talking about here in the UK. There is no legal requirement for Covid testing in any scenario here . None whatsoever (certainly not in England - I don't keep up to date with the other nations but I believe they are the same). Some people being admitted for hospital procedures may require one so that their treatment can be properly determined (in the same way some are tested for other ailments such as blood disorders or diabetes). The NHS is advising patient-facing staff to test themselves twice weekly even if asymptomatic. I can (just about) understand that though with most of the country already having been exposed to the virus, and/or having been vaccinated, it seems a little pointless. Other than that, I'm interested to learn why some organisations are still insisting on it. Nobody seems to know why.
If I go abroad I comply with any requirements that country makes of me (or, if I cannot or will not comply with them, I don't go).
//Is this question really
Do I have to comply with my employers instructions?
answer : yes if you wish to continue to be employed.//
Answer - not if the requirements can be shown to be unnecessary, unreasonable or disproportionate (to the perceived threat). I would suggest that, in most circumstances, insisting on an uncomfortable medical test being taken to detect a widespread and, for most people, a not particularly serious disease, is all three of those. This is somewhat different because jadj does not have an employer - he is employed by himself. He has customers or clients.