So, in 1935, the 'Report of the Archbishops' Commission on the Relations between Church and State' despairingly wrote:" No one obeys the law so construed [by the Privy Council in 1868]. Not the clergy, since there is scarcely one of them who makes no change in the authorised forms of service. Not the Bishops, who are charged to see that this impossible law is carried out. Worse still, by the Declaration, as interpreted by the Courts, every priest solemnly undertakes to do that which in fact none of them actually performs."
After the war, Archbishop Geoffrey Fisher, a resolute persecutor (while Bishop of London) of his Roman Rite parishes, masterminded the provision of a completely new Code of Canon Law. This cut the gordian knot which the Privy Council had tied in 1868. It decreed that "the minister who is to conduct the service may at his discretion make and use variations which are not of substantial importance". "Trivial" rules, OK.
Where does this leave us now? Most bishops steer clear of getting into liturgical wars. But there is a handful (curiously, mostly 'Catholic') which takes the line "My dear boy, I'm prepared to turn a blind eye to pretty well everything you do, and I'll even go along with it when I visit your parish. But just one thing: you MUST say an approved Eucharistic Prayer after the Sanctus." (I know one episcopal fool who commends this approach by saying "I've got to persecute you so that I can persecute the Evangelicals too".)
One can see a reason for this. The Eucharistic Prayer, in recent Anglican history, is rather like the Ypres Salient; every yard doggedly fought over; gallons of blood spilt; whole acres dangerously mined. The 'approved' Eucharistic Prayers are supposed to have been so crafted that not a syllable treads on any toe, whether it be a Zwinglian or Transaccidentationalist one. (Gregory Dix foresaw these hypocrisies: " There is really something very profane about the idea that we can only come before God with circumlocutions which it has been agreed to misunderstand differently".) The only fly in the ointment is that Canon Law elsewhere makes it clear that the same degree of regulation applies to every part of every (legal) service; the notion that the Post-sanctus is a unique and privileged area can find no justification in the text of the canons.
It would legally be possible to test whether certain 'variations' were contrary to 'Anglican doctrine'. But if it were to be argued that - for example - the Canon Romanus was contrary to Anglican doctrine, there is morass of quicksand that the troops would have some trouble getting their heavy armour across. A few years ago the Anglican bishops, while nagging the RC authorities to allow 'intercommunion', said that Anglicans ought to be admitted to Roman Catholic altars because Anglicans "can, with good conscience, say a heartfelt Amen at the end of the [Roman] Eucharistic Prayer".
These ad hominem arguments can get you into such a fix. You can't have things both ways. Not even if you're a bishop. Not even if you're an Anglican one.