Even in "modern" times, it can take quite a while to get from say Plymouth to Inverness. I remember my mum (Devonian) recalling how long that trip took back in the '60s. Then she got posted to the islands on the north left and getting back home really took some time - the ferries can be a bit sporadic when the weather gets grotty 8)
Try not to look at the map of Europe as a driver of a vehicle capable of >100mph. Imagine there are very few roads, except what the Romans left behind. There are not many people and a lot of trees. Police is a concept that is yet to be invented although the Shire Reeves (Sheriff) is an early model. Most people's horizons probably don't stretch much more than 10 miles for their entire life. Witches are real and doctors were scarce and scary - they hadn't even invented the leech at this time.
That said, some folk did travel quite a long way. Bede famously recalls a story of a pope who remarks on some slaves in a market in Rome: "... they are surely Angels ...". He was talking about some Angles who were taken from these isles who he found rather good looking.
If you really want to get a flavour of just how complicated things are genetically here, let alone in the rest of Europe, try Stephen Oppenheimer's "The origins of the British" (1-84119 894 3)