Alex Ferguson is Scottish and I dont think any Scottish manager could bring himself to manage England.
Plus I doubt if the FA would ever ask a Scotsman to manage England.
Also, he is a successful manager at Man Utd, well paid, well respected, winning trophies.
Why would he want to leave?
Finally being England manager is a poisoned chalice. Whoever gets the jobs gets called all sorts of awful names by the press and their private life is ruined, so why bother.