At this point it's less of an issue in technology – and just an issue of sample size. Genetic variation is how a species survives – each child born is not just the combined halves of their parents, they're a unique mix of each with some fresh mutations thrown in. So it is a question of getting enough of a picture of the whole to be able to determine which is normal genetic variance “noise” and which is unique at the population level. In order to have any hope of being able to finely map out our migration patterns using genetics (and I actually doubt this is possible at all, we're just too similar) we have to have a massive, massive sample collection from all over the world. Kept in a single database, with a dedicated team chasing down patterns. At the moment that simply doesn't exist. But even if it did, I'm not sure you could get a clear picture of invasions/migrations from two groups as similar and in close contact as the Indigenous Brits and the Anglo-Saxons. Genetically, they're pretty much the same people. What distinguished them was their culture. Does that make sense?