I think the likelihood of a "Celtic Aliance" is a bit far fetched. It's certainly possible that Scotland and Ireland would fall away from the influence of Westminster, but Britain had a lot of Cold war contingencies for maintaining control, I honestly couldn't see England fracturing that much, nor could I see the southeast being controlled by the Irish Government.
Even so, that Lancastrian flag looks awesome!

EDIT: although now I think of it, the lesser affected areas would likely stand a greater chance of survival than the major cities and the capital, and would probably assume far greater autonomy if not total independence from the surviving central government.