I think they label it as that bc in the far reaches of history, Great Britain in particular had a lot of genetic influence from areas which now constitute Northern France, Germany, The Netherlands and Denmark. Because of the mixing of populations they can’t always concretely say that someone is from the South East vs someone from Holland due to genetic similarities that can occur.

As much as I know the Irish hate it, there’s not a massive difference genetically between the British and Irish, so they can’t conclusively split them into two and separate it - again due to history and the mixing that came with it.

Maybe in the future they’ll have the technology to be able to split the countries up more accurately but since DNA testing is still in its infancy really I think they err on the side of caution by putting the broadly category in there to account for pieces of your DNA that they can’t specifically pinpoint to one region since it may look similar to the DNA found in nearby countries.

Sorry if I butchered that explanation!