Besides possibly the Breton War of Succession, what other scenario would allow for Brittany to become part of the United Kingdom?
In the middle ages, that's rather easy. Just change some marriage, kill the odd heir and voila. The problem is to keep Brittany in the nited Kingdom. For that you'd either have to increase the continental holdings of England significantly so that the additional power base is large enough to hold, or yo'd have to destroy France as an entity, maybe having France balkanize like the HRE did and working as a buffer between Brittany and any larger power. In any case, changes are so profound that it's very unclear whether a United Kingdom emerges and what constituencies it would encompass. For example, assuming that England must be in it, it's rather likely IMHO that the much increased focus of England on its continental holdings weakens it enough for Scotland to remain independent.
Have Edward V continue to reign. He was engaged to Anne of Brittany. There you go, personal union between England and Brittany and an eventual integration.
Because, of course, it would never change hands again, never be taken by France, and never rebel. Can we at least pretend that those things are possible?
Just for discussion's sake?