Hi T_B_S, there certainly are PE seats on the main deck of the VS LHR 744s.
I didn't count rows but something like about 15 rows of UC, then a dozen or more of PE before Y. The PE cabine went well into the wing root area.
Upstairs is three of UC then six (I think) of PE. My mother is going to be there tonight (28th) on the JNB. I suggested that she shold ask for a PE seat in the top deck as it is so much quieter. At age 77 she is still spry but dislikes children almost as much as I do.
I hope youo enjoy IAD, I am hoping to get there next year for the first time.
Cheers,