I'm afraid Twilight check in and exit seats doesn't really work now.
Now that the airport operate the twilight check in (and not Virgin staff) they cannot issue exit seats at this time.
If you request an exit seat your name will be put on a list, and the next day at the Virgin ticket desk (not check in) exits seats will be issued on a first come first served basis.
I thought it was rubbish too, in fact so rubbish I upgraded to PE just to guarantee the big seat (I am 6ft 4).
Hope this information helps,
Julian.