Substance we can evidently sign away, as long as the economics add up. This is not always a reliable test, by the way. The BMW takeover of Rover ended in tears, and foreign-owned banks may survive a recession less readily than if British interests still controlled them. But we blithely swallow that. Symbol, on the other hand, must be defended at all costs.
Passports are a symbol.
Parliament itself, while capable of its historic tasks, now seldom performs them. Under all parties it is a tool of executive government, just as all countries face interlinked complexities of political and economics that none can alone control. Though parliament can't be destroyed by the EU, it is in truth as much symbol as reality. One question now is where in the British mind football is allocated. If Mr Eriksson wins the World Cup he will become an English hero.