Seat Allocation

Please forgive me a I expect this question has been asked many times before.
I will be flying for the first time with Virgin in January LHR-MIA returning in February LAX-LHR both flights in PE.
When can I book seats and is there a charge?
Can these seats be changed by Virgin staff before check in? If so is it best to wait until check in and see what is available?
Are people flying on the same booking reference seated next to each other?
I will be flying for the first time with Virgin in January LHR-MIA returning in February LAX-LHR both flights in PE.
When can I book seats and is there a charge?
Can these seats be changed by Virgin staff before check in? If so is it best to wait until check in and see what is available?
Are people flying on the same booking reference seated next to each other?