How can a pastor uphold good doctrine in a denomination that doesn't?
You certainly want to help your church get clarity. You don't want them to be confused. Probably staying there long enough and teaching in a faithful, biblical way will remove the confusion and replace it with conviction. And you should draw encouragement from other ministers, wherever you can find them.
I know that in England there are hundreds of faithful evangelical ministers. They relate in various groupings, like the Proclamation Trust, and others. And England is not a very big place.
But mainly, stay close to the Bible, stay close to God, and love your people. Teach them so faithfully that they recognize that what other pastors are denying is clearly a denial of Scripture, not just a denial of their pastor's opinion. The key there is going to be that, week after week, he doesn't just preach Reformed theology; he preaches the Scripture in such a way that they infer Reformed theology.
I am sometimes sent tapes from other pastors to listen to, to see what I think. I once listened to one that helped me realize why Reformed teaching doesn't make headway in some places. What he had done was take the first 20 minutes of his sermon to lay out all of the pros and cons about some doctrinal issue of Reformed theology, and only near the end did he even come close to the Scriptures. And I thought, "No way! Never in a thousand years will a church flourish under that kind of preaching!"
You don't start with doctrinal statements taken from the Westminster Confession—giving three reasons for them, three reasons against, and talking about all the historical controversies—and then tack on a text at the end as a proof text. You start with the text and you unfold it for the people so that they see, right there in the word, what is true. The Bible is shocking in what it says, over and over again, and you just need the Scriptures.
So labor in faithful exposition, week in and week out, loving the people, so that their confusion is taken away and their conviction grows. Then they will be your partners and advocates as you deal with other ministers in the area.