I just got my results and had the same questions as you. Blond hair, blue eyes typical Anglo-saxon stock (with some Norman and Viking thrown in there), and I can also trace most parts of my family back to >1500, including Paternal line (Smith!) which is the one that has thrown up M-172 as a result.
I was also sceptical that maybe there was a fault, but then I looked into the fat there are a fair few M-172s in the UK, and started to research a bit.
I've looked into it some more and have three theories:
Most Likely: ROMANs did it. Someone from the Med came over with the Romans and impregnated a local, or settled. Quite likely actually. This was afterall some thousand and a half years before I can trace my Smith family, hundreds of years before surnames started, and loads of time for the mediterranean traights to be bred away and replaced by anglo-saxon. This is a very reasonable explanation, the Roman influence on the UK was massive.
2nd Most Likely: PHOENECIANs or Someone else from bible-country coming over on a Phoenecian boat. It's pretty well known that there was extensive trading between them and the british isles, especially for tin. All those legends of Joseph of Arimathea coming to the UK etc etc.
3rd option (starting to get quite unlikely..): Accoriding to ancient British (welsh) legend there were two major migrations from places where M-172 is from during pre-biblical times. Firstly - from UR (Syria), by a King Albin (see. Albion) and his followers some x000 years ago. Secondly, from Troy (Turkey) led by Brutus (see. Britain). Either could have brought the bloodline. But this gets into the realm of legend and lots of dodgy pseudo-history. The most compelling things I could find about this were written by Alan Wilson and Baram Blackett, who also claim that this is the line of king arthur etc... so go figure! However, then I can't explain why such a thin M-172 thread should survive in amongst the much more broadly used British/Welsh/Celt haplogroups, unless they were some kind of ruling elite...