Hi James,
A brand search is likely to throw up international pages as it is very different from a search term - this is much more specific and handled differently because you are using sub-domains rather than a folder structure. This is (almost) another separate entity in Google's eyes.
It is possible that over time Google will address this, but I am not sure there is anything more than you have already done that will make more differences.
Have you done a new fetch as Google? Are you sure HREFLANG is all working correctly? You can use this to test it.
-Andy