I think you would be exempt from restrictions if transferring in any of those three locations. UK regards Greece and Iceland as exempt from restrictions (though there is an online form you must complete even if staying airside) and you get a security check which the other two locations don't have. Transfers aren't greatly restricted in the UK even if the country is not exempt. If the cost is equal I would exclude Hungary simply because it has created its own entry requirements and changed the rules many times, but not to the extent that affects your plans. Vienna seems the easiest overall. But no testing is needed on those transfer locations. Welcome to Flyertalk.