Ancestry's customer service is pretty clueless about DNA, but it's not really their fault. They're bearing the brunt of Ancestry's latest foray into genetic genealogy, but they clearly haven't been adequately trained or educated on the topic. Ancestry has to take the blame for that and the obvious shortcomings of their autosomal DNA product.

Also, keep in mind ethnicity isn't really something that can be inherited. It's transmitted by your family and the environment you grow up in. Some people have legitimately found the heritage they've identified with their whole lives isn't correct. In your case, if your great grandparent had 100% British ancestors, you almost certainly did inherit DNA from them. That's not many generations removed.