May help to separate the three processes
- Avios have a 24 hour cut off - longstanding policy - which allows a refund minus a redeposit fee.
- FTV for Avios bookings (and eVouchers for commerical bookings) - let's say 2 hours from departure though you can go a bit later perhaps. That's under BWC, so pademic related.
- Sickness policy - also longstanding but shrouded in some secrecy. We know it exists but BA staff are fairly coy about it. In essence though it does allow for someone too ill to travel to defer travel to a later date. The idea is to stop people with bad flu forcing themselves on to a flight due to a non flexible booking. Generally requires some verifiable paperwork.

So I guess you're waiting for a PCR test result and the timing may influence which of these three routes is best suited.