Rapunzal,

As per your monarch flights, it's personal preference which part of the aircraft you like. Your flights are operated by a Boeing 777-236/ER. For a seating plan, click here or here. Seatguru (second link) gives a bit more information on specific seats. BA don't allow pre-booking of your seats more than 24 hours before your flight departs. That's why you can't book specific seats this far in advance. For more information click here. BA have announced the move of longhaul flights to T5 has been postponed. They are planning a phased move to T5 starting June but you'll have to check closer to the time of your flights if they have or not. I would expect (hope!) they will have done by December. Click here for more information.

Darren