If you start talking about salaried work, you have to account for Austria's 5 weeks of mandatory PTO, infinite paid sick days (with a pay reduction after 6 weeks, clock resets each year), additional paid care leave (Pflegefreistellung) for sick family, paid maternity / paternity leave, etc. in contrast with whatever the British system offers. For developers in Austria who choose salaried work, the math tends to add up with at least 2 months off each year, union protection etc.

You'd have to be an expert on labor law in every country you seek to honestly compare. The only clean way to simply compare earnings is freelance rates.