Can Eng pick players not in the EPS or SAXONS

If you were gonna pick on form it's pretty clear to every keen observer of the Prem that Sam Dickinson is worthy of at least a place in the EPS squad if not the starting 15,

However With international packs getting bigger I think there could be a place for Bully Vunipola at 6 where he's been playing for Saracens!

Is it possible for Lancaster to pick Dickinson?

Yes, Lancaster can pick any player he likes (who is eligible) to play for England. What EPS/Saxons gives him is:

a) Extra access to players
b) the right to manage a players workload.