Not sure about boarding. I think it might vary from station to station. London can be a mess quite frankly as perhaps you've discovered. The website says the screens are 8.4inches in both - but it's funny I thought they were bigger in Elite... In Elite they are in the arm and pop up. In Economy they are on the seat back (except where there is no seat in front, then they attached one to your arm). But for Elite if you have a laptop is that there is a power socket (110V Euro/US socket) so you can watch your own stuff.

The film choice is slightly less in Elite. The system is basically the same, but there are always a couple of films that are PL only, and a couple that are PL/Elite only.