There are British companies that sell (among other things) flour made from Canadian wheat.

The specific one whose flours I use for my (very amateur) bread baking is Shipton Mill. I tend to make my bread using some of their very strong Canadian flour and some of their others which are weaker but possibly produce more interesting-tasting bread. (I do not guarantee that I would actually notice the difference in a blind test. I haven't tried.) One of the other flours I use is made from some continental-European and some British wheat. Another is all-British.

(I am not personally all that concerned whether my flour-buying supports British businesses; I'm just one person making a small amount of bread for my own family's use and the impact of my flour purchases on the national economy is a rounding error on top of a rounding error.)

As others have said, you could try using not-Canadian wheat and adding some vital wheat gluten. I haven't tried doing this myself but my understanding is that it works about as well as using stronger flour to begin with. It might make your ingredient lists a little less appealing if you're going to be doing this commercially, though.