I have been researching my family history for over 30 years. I know who all my 16 great great grandparents were. All of them were born in England and came from families who had been established in England, in some cases back to the 16th century, before which paper records are hard to find. The 10% English is probably from one line, who were established in North East England for centuries, but I know are all descended from one single person, who was born in London. My birth name is very unusual, so it's been easy to trace.
Of course, it's always possible that a father isn't the person who is stated on a birth/baptism certificate. However, I have done all my own research and haven't copied and pasted other people's research, as some people do. I am confident that my family tree is as accurate as it can be and I have always identified as English to the core.
I was given an Ancestry autosomal DNA test as a present a couple of years ago. I didn't expect it to be that accurate, but the results were totally unexpected. Apparently, my DNA is just over 10% English with the rest split almost evenly between French and northern European (mainly German). It was so unexpected that I even queried it with Ancestry because I thought I might have been sent the wrong result. The reply explained the limitations of the test.
If I hadn't already gathered so much information about my family tree, I might have been looking for French and German links in my family, but I know there aren't any, so where have they come from?
The reply I received from Ancestry suggested that my paternal grandmother, who was born in Devon, could be the link to French ancestry from Britanny, as well as some of my paternal ancestors, whom I already knew were probably immigrants from Normandy in the 16th century. It's possible that some of them had northern European (Norse) DNA.
The German ancestry is probably because most of my mother's forbears lived for centuries in Anglo-Saxon areas and they have passed on their Angle/Jute, Saxon and Danish DNA for over a 1,000 years. I doubt if any of them ever identified as German.
An autosomal test only tells you where most people with similar DNA live. It doesn't tell you anything about your personal ancestry. In my case, people with DNA most similar to mine still live in France and northern Europe, which is why I received the results I did. However, that doesn't make me half-French or half-German or any nonsense like that. I'm ethnically English and there isn't any doubt about it.
Men's DNA tests give more personal information because men have X and Y chromosomes. Women don't have Y chromosomes, so can only trace their maternal line directly.
In the case of Welsh ancestry, which snapshot in history do you use? Many Welsh people are descended from people who originally inhabited England. Scottish people might very well find that their DNA is shared with people in England or Canada or other places. It doesn't mean that they are descended from people in those places, but Scots have emigrated and established communities who share DNA, so a simple DNA test can be misleading.