I have been thinking about it more and in fairness I've realised the fault probably doesn't lie with the S&C coaches.
Generally in off-season/pre-season players will basically live in the gym, but once the season hits they can only really lift for 2, maybe 3 days a week (tend to need 2 days after the game to recover and 2 days before a game to make sure they're fresh and firing).
Essentially, they build all of their strength before the season and the rest of the season is just a race against atrophy to maintain that strength and muscle mass.
The issue for England players is that they don't get an off-season/pre-season. They go almost straight from summer tour into the season, and so they never have a period of time where they can actually make any serious progress. They're constantly just in that race against atrophy, hence why we often see such a big difference in players who have had long injury lay offs (Curry, Watson), because they have a period of time away from the game where they can actually focus on physical development.
But players in teams like Ireland get regular rest during the season, and so have periods where they can focus on physical development.
So, like so many others things, I think a big part of why our players aren't as physically developed is because they're being severely overplayed.