This is very usefull information for the Austrians I am helping. But it seems impossible to get from Spain to Austria. Flights from Spain are not allowed to land in Austria and transiting Spain, France and Germany to go to Austria by train or car seems impossible and high risk to catch the virus. As you say it sounds stessfull or maybe impossible. Maybe in a month time with infection numbers going down in those countries.
There are no restrictions on travel into the UK, so I think you could get to Spain then to London OK. Not sure about the London to Austria part