Mike, why are only a third of English league players English, and why are half the Belgium squad English club players.

we should remember that Many British players played abroad

Not many compared to the reverse flow.
It may not be a wholly one way traffic, but almost.

English players earn very high wages compared with most Foreign leagues.

That applies to bought in players too, plus the transfer fee.
Do other European leagues pay significantly less?

Why do only English clubs seek to profit from selling players?
Why does England produce so few elite players from its large population?
Why do English clubs prefer to buy in players than train English kids?