All very well talking about seating plans and the differences between them but at the end of the day it wont make any difference for 2 reasons:
(1) There is no guarantee that you'll end up on an LGW 744 as opposed to an LHR one because there are so many a/c changes at the last minute that a/c originally slated to do a particular flight end up going somewhere compltetly different
(2)Even if its the same a/c,you can't request exit seats anyway.They are given out at the gate at the last minute on a first come/first serve basis.Quite frequently staff will end up sitting there if Upper/Premium are full.
I should know,being an ex VS employee!