Are you entering this under the perception that this is post apocalyptic?

Are you implying a feudal system is an expertly crafted system of stability? Cause, it's not. It relies on the opinion of your underlings with little to no administrative oversight to check and balance the dukes and counts to reel them in if they get uppity. And medieval arms are impractical in this world. Not saying you can't use them, but when the armies of sheep-husbands move on the British I imagine they'll be gunned down, or will be gunned down by any ally of the British power. One helicopter could lay their armies to waste.