I don't think Great Britain really makes sense as an ethnicity. Ignoring recent arrivals, there are three ethnic backgrounds in Great Britain. The Scots and Welsh are primarily Celtic and are presumably more closely related to the Irish than to the English. Then there are the Angles and Saxons (Germanic) who make up most of the English population. Finally, there are the Norman French who arrived in 1066. The Franks were a Germanic people, so I don't know how genetically distinct they are from the Angles and Saxons who came from Germany.

If English descendants of the Angles and Saxons and Norman French were classified as "Germanic" along with people from Germany (and probably the Netherlands, Belgium, and Northern France), my AncestryDNA results would make sense. Calling the majority of my ethnicity "Great Britain" just throws into question the credibility of the results.