simple fact that you have an island so close to the mainland (Brittain) developing means they'd have dragged the rest of society out of the middle ages significantly earlier.
Making them isolationist and xenophobic would explain the fact that slavery was still alive and well, and also explain why the Roman technology hadn't fallen into the hands of France etc. If you move it back a few hundred years perhaps they had also enslaved and absorbed the vikings who in this reality of your game would be nothing but a minor historical footnote...
Food for thought...