If the Welsh are descended from the tribe of Dan, then they would be related to other colonies of Dan. There was silver mining in Spain at the time of Dan's shipbuilding. This would've been ~1000 BC. The Anglo-Saxons took over Britain in ~500 A.D. for a difference of 1500 years. In those 1500 years, the Welsh would've mixed with some of indigenous people. Also in those 1500 years, the Isrealites (Anglo-Saxons) would have went into captivity by the Babylonians, the Assyrians, the Medians and Persians, made their way across the Caucusus, mixed with the indigenous peoples all the way to Britain. So there would have been differences in genetics. That doesn't mean that Dan never made it to Britain in 1000 B.C. Look at American blacks today. Almost all of them now have English blood in them because of slavery and mixing, but could you do a study on them and conclude they come from Friesland, who are closely related to the British.
Hmmmm. There were worldwide 'Dark Ages' at 1159BC and 540AD.
Oopsie. The Southern Kingdom which went into Babylonian captivity were not Anglo-Saxons. The Anglo Saxons came from the Northern Kingdom which went into Assyrian captivity, and upon breaking free burst onto history as The Celts. (For Details, please click on my Profile, below.)
But they all were of course cousins, and we assume all 12 tribes more or less freely intermingled up through the time the Davidic Kingdom split in 922 BC.