I think that the English national side would not gain anything from less foreigners in the Premier League. I doubt that the likes of Gerrard, Lampard, Terry and Rooney would have been so good unless they would had played against players like van Nistelrooy, Henry, Bergkamp, Stam and Ronaldo.
If you use the 6+5 rule some players might become slightly better by playing regularly against worldclass players, but they won't go from mediocre to worldclass.
Instead you should try and improve your academies. Many english footballers say that there was someone better than them when they were kids who gave up the sport. You should try and get this "someone" to continue.
In Sweden there was a kid that that wanted to quit football. His coaches managed to convince him to continue. His name was Zlatan Ibrahimovic.
And I would also like to say that the reason for that England haven't won anything since 1966 is bad luck. Just look at Spain, they just won their first title since 1964. England's time will come.