Focusing on skin and eye color seems imprecise and really, why even bring it up?  DNA mutations or markers spread in a population.  Then some members of that population travel to distant lands and intermarry (or not, maybe just die,) and spread their DNA mutations around and then die.  Hundreds of year later someone sequences the DNA and says, hmm, here's a marker for X population but I found it in territory Y indicating that people moved around or were taken as slaves so there was a lot more mixing than history expects.   
   Does your theory about the original Britons imply some direct-from-<some continent> migration?  If so you'd expect markers for populations on that continent to be present in historical samples without the associated markers associated with a migration via a more lengthy route.   So let's test for that which will either prove or not-prove the theory and leave the silly adjectives for twitter.