Isn't the TIR convention mainly so that containers, etc, don't have to be opened up when passing through countries on the way to other countries? (I was baffled in my youth to see that many lorries bore the word "TIR", meaning "Country" in Irish, and a trucker explained this to me.

I don't think TIR means duties don't have to be paid by exporters, which is how Britain is treating these goods at the moment (as far as I know).