Hi Botham,
A few thoughts below.
1) An edge case but sometimes the ticket type is sometimes important, whilst this is a scheduled BA flight, such companies may book as a group. In my experience, I was never able to pick a seat in advance of arriving at the airport, indeed was not able to check-in until the airport. I guess it is to do with the same PNR and PII issues. This is the worst case. If you can MMB now and are allowed to pay for a seat change, then the above does probably not apply to you.
2) If you are on a regular ticket (most likely), as you know you can book T-7days and silver at any time after the ticket is issued (anything available except row one).
3) GCH and above get exclusive access to row one in advance of T-72. Although I don’t think this route will be full of GCH (although maybe I am wrong about that).
4) If a GCH has not booked row 1 by T-72 Hours it will probably open up to Silver and Bronze.
5) There is an exception to the above (4), lets say there is a GCH in 1A then theoretical seating may kick-in at &-72 (on a non-full flight) and block 1C. The same with 1E 1F examples.
6) To be honest most of the time, I am lucky and manage to get a row one seat as Silver (and previously as bronze).
Other bits:
7) The size of CE may expand or contract based on load (moving curtain). If this is more about you sitting together rather than specifically in the front row, keep you eye out for new rows becoming available where you can pair.
8) There is a nice little catch on some A320 (although I think out of LGW rather than LHR) where E and F doesn’t start until Row 2 (bulkhead at row 2) and therefore open to Silver and Bronze in line with their BAEC benefits free seat selection times.
9) Finally I don’t think BA fly 767’s anymore.