By way of a quick update, which might be useful for others:

I used Qured video tests for the 4 of us which were really good. I tried Breathe but they didn't have appointment availability. Results came back within minutes for two of us and a few hours for the other two.

VeriFly worked really well, however BA would not let me OLCI and had to do it at the airport, however there was no need for any paperwork at the airport and lots of other pax having to produce paper.

At 4 days from departure seats appear in MMB for the longhaul so I didn't bother calling to review as they were good seats.

MAN was a cattle market. The checkin queue for the first LHR and LGW flights at 07:00 were massive, however the J/F check-in desks were empty so we were straight through.

The security queue was massive and whilst we were checking in had snaked passed the BA check-desks and looked set to end up outside. This was before 05:00!

Fast Track security was bliss and we went straight to the front - with only about 5 people in front of us.

Escape Lounge in T3 was fairly quiet, although the staff at the entrance were turning away people without bookings or who weren't booked on an eligible airline/ticket.

We arrived airside at LHR which I wasn't expecting? I was expecting to have to go through security again, so that was good as well!

Thanks for everyone's help! I hope my update helps others.