Thanks--I am reading more also, and have consulted a specialist in genetic genealogy. She characterized the Ancestry test as "wide but shallow," that is, it is primarily useful for identifying or confirming fairly close relationships. However, the recombination of DNA over many generations can obscure origins. She also offered the opinion that some of my German ancestry was Saxon and was included in the British Isles category, a conclusion that is supported by the fact that several of my German ancestors were from Lower Saxony.
Hopefully there will be more detailed results eventually. In the meantime, I ordered a Y-DNA test from Family Tree DNA and we'll see what that reveals.