Once you have downloaded your boarding pass, you can't change your seat allocation - that can only be done by staff at the airport, if there's some reason they need to move you around.

The X's are seats which aren't reserved, but which are blocked from being selected (hence they show 'unavailable' in seat selectors). This blocking can be for a variety of reasons, but in this context it's generally either because they are nominal seats that BA has kept free for passengers who haven't yet reserved a seat (to ensure families can sit together, for instance), or because they are Executive Club Gold status holders and so the system will try and keep a seat free next to them where possible. The latter is known as Theoretical Seating and has been discussed at length in other threads.

These seatmaps are from Expert Flyer.

The last sentence describes how many tickets BA is willing to sell for your flight in different booking classes - generally, the price decreases the further along you get in the sentence.

F/A are first class, and only 1 seat is being sold, under the most expensive class, F. J/C/D/R/I are business class, and BA isn't selling any tickets for this. W/E/T is World Traveller Plus (premium economy), with 1 seat available under the most expensive class, W. Y all the way through to G is economy, and BA is willing to sell up to 3 seats in Y (the most expensive), or 1 seat under H, K, or M class.

All of which is to say, BA is currently only willing to sell 1 seat in First, 1 in premium economy and 3 in economy. Considering the fact that most flights are overbooked to a certain extent, this suggests it's going to be a virtually 100% full flight.