In their promotion the DNA testing companies have heavily promoted the ethnicity aspect of the test. As the article you linked to shows it's not accurate for distinguishing between supposedly different groups of Western Europeans. Most Brits will learn nothing from it that they didn't already know and may even be led up the garden path.
The strength of the Ancestry test for genealogists is the tree links to 4th/5th/6th cousins which can and do help break down brickwalls. But it depends on the people testing making and sharing accurate trees. You will see many matches to people without trees who just took the test for the ethnicity, never followed up the family tree side of it and haven't logged in since. Which is somewhat frustrating.