The LGW/MAN fleet is made up of eight B744's - five (VGAL, VLIP, VROM, VROS, VROY) have the Nova entertainment system and the other three (VAST, VTOP and VXLG) have the Odyssey system.
There is a small chance because of operational reasons that an LHR B744 could operate the flight however this is a very small chance. Usually when an LHR B744 is at LGW, it operates the BGI route.
You be able to find out which aircraft you'll be travelling on by looking out of the window when you arrive at the gate for boarding.
And you are correct to assume that the LHR B744's do not have the extra legroom seats but you would still be able to purchase exit row seats on the day as long as you meet all the requirements to be able to sit there.
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