So you need to check in online but not collect your boarding pass (you get to a page where it offers you options on how you can collect your boarding pass - just ignore that page and close the app/browser). You are still checked in at this point.

After this, you can go back into Manage My Booking and change seat like you would do if you were not checked in. Occasionally you can move seats even after you get to the airport and collect a paper boarding pass, however this is likely to not be possible on this flight.

The X means a seat is blocked. This, on a full flight, means that the seat is generally allocated by the system, but this can be overridden under certain circumstances (e.g. status passengers). There are likely to be more blocked seats shown on the seat map above than on BA.com for this reason (unless one holds status).

The information shared is on a website called ExpertFlyer, and is available either via trial or paid subscription.

If you don’t want to be stuck in a middle seat I would strongly recommend paying for a seat (you get to see what you’re paying for before you pay).

Hope you have a safe and comfortable flight.

p.s. the letters and numbers are a snapshot of the seats on sale for your flight, they provide some indication of flight loading, over sales situations and ultimately a view on whether you will in your case have some spare seats in your cabin on not. In your case I think the answer is likely to be no I’m afraid.