I did a bit of research after the Iceland game a few years ago. The squad at that time had somewhere in the region of 3,400 Premier League appearances between them and they lost to a country with, as Lineker put it, "more volcanoes than professional footballers".
Premier League experience...
It's also worth noting that Chelsea, and many other clubs to be fair, harvest young English talent and then farm them out to the Championship and below. They'll probably never get close to the 1st team but if they end up in the Championship maybe Southgate will spot them.