My guess is that - until the day, the airline do not know either. In my field of work, Covid has laid low too many people and every day there are calls to say that someone cannot come in as they have tested positive or have symptoms.
The NHS has the same problem, a friend of mine who works in a GP surgery said that eight people went down with Covid in one week. No company can plan for that kind of staff outage. My brother and his family had booked for Cirque du Soleil at the Albert Hall. Made a weekend of it with family from Devon and Nottingham. Got to the door - Sorry but too many of the backstage crew and performers tested positive, you'll get your money back. Two grandchildren rather dissapointed.
That is what the UK is currently dealing with. No company, particularly one as large as BA with such a range of staff all needing to be fit and ready to go - can promise much these days. We travelled long haul with them just over a week ago and, apart from a delay whilst waiting for cleaners to complete their work on the A380, all went fine.
We were one of the last out on a Friday night, so I was not surprised at the delay. Flight and Cabin crew all fine as usual.