Austria has banned flights from the UK and several other countries so your flight won’t be operating unless that changes. But BA ought to switch you to MUC if you still want to go.
The test certification Austria require is specifically this document completed by a doctor.
https://www.ris.bka.gv.at/Dokumente/...1752893.pdfsig So you’d need a test from a willing private GP or perhaps a friendly GP would do this after a government test.
Further info and perhaps updates here.
https://www.austria.info/en/service-...us-information