Possibly stating the obvious, but the turnaround on a PCR test is a day or two, so putting it in right away would be rather challenging.
I've been on to the bank as BA have charged me twice (one is pending) but that is now sorted I hope.
I had a chat with the friendly lady and told her the story and mentioned about the PCR tests coming in, she mentioned a lot people are disputing transactions as they did not receive the test results in time as advertised (I assume they are flying abroad) - so beware. She also mentioned someone had to stump up £4k for another flight because of this. So if you are planning a flight in or out, be on your guard. I doubt you'd be able to get insurance for this sort of thing.
Also guess what, the 'cancelled' flight last night actually went ahead. Must have been a fresh air 'positioning move' more than anything. Been on to BA, I should still receive a full cash refund (no vouchers!), if something goes wrong -which it will- I'll start a new thread. Not holding my breath on this one. It's a good job my oven is electric and not gas!