LOMCEVAK
Incorrect. Peacetime clearance for F3 was 725KIAS Vno and 750KIAS Vne. There were various speeds above that for operational necessity, under Operational Emergency Clearance (OEC), one of which was 850KIAS for F3. I did hear that 900KIAS was also authorised at one point and I do know some that got to that with ease before the DECUs were trimmed back following some uncontained engine issues.
We still do OECs these days and you can read about their policy here:
https://assets.publishing.service.go...30_Issue_5.pdf