England 1453-1486
Basically unification, and suffering through the Wars of the Roses.
Unification
The previous ruler (one Bocaj, a man who clearly hates England) had allowed 5 separate single-province kingdoms to arise in Ireland. Only one of these, Eire, was an English vassal and ally. England had no claim on any provinces other than the 10 main provinces of mainland England, and CB's only on Scotland and, mysteriously, on our vassal Eire. Unification would require the assimilation of 6 separate countries.
Celtic forces outnumbered England 3:1. Scotland was allied with mighty France. Each had military ability equal to, or greater than, England. Englands technological ability had not advanced at all, again mysteriously, in the years since 1419.
The question was whether to try to bring the Celts under our protection diplomatically, or risk the disapproval of world (well, AI

) opinion, and low stability within the realm, by conquest. We were sure that diplomatic attempts would be very expensive, and often fail, leaving a permanent and expensive thorn in the side of England. So it was to be unification by the sword.
I was not going to die with Calais engraved on
my heart, so a deal with France allowed Calais to return to the burghers for a pittance, in return for unhampered conquest of the Celts.
We married a daughter to the Red Hand of Ulster, claimed the throne, and having reassured my own people that Ulster hated us (by insulting them many times) used this as a pretext to conquer them.
We hoped this would rouse the ire of the Scots, but they remained passive so, shunning weak-kneed diplomacy, the army was sent to claim all of Scotland other than its capital.
Our vassal ally Eire fortunately declared war on Leinster, so we were quickly able to take control of military operations with our superior leadership, and annex it.
The naive Scots had accepted a 5 year truce - 5 years and one day after that expired, the army marched into Edinburgh and soon after Scotland was ours. Possession and trafficking in haggis was immediately made a capital offence - let them eat proper English sausage.
Connacht and Munster foolishly allied with one another against the English threat. Another daughter was sent to sacrifice her maidenhood to an uncouth tribal princeling of Connacht, the throne was claimed, and after more insults were hurled at them, they too were annexed, leaving only our vassal Eire independent.
At this point stability in England was continuously low, and there was some risk of revolt as the English aristocracy were busy murdering one another to decide whether the Red Rose or White should rule the realm. And world disapproval of England had been rising continuously. So some fine calculations were required for the final decision about Eire. To wait for 17 more years, and attempt to diplomatically annex them, with all the risks of failure, or undertake one last conquest? Insults were sent, military access cancelled, they were thrown out of their alliance with England, and the troops marched one last time. Such high-handed treatment of a vassal caused consternation in the realm, but the few revolts were easily quelled. After annexation, world opinion was as close as possible to the maximum it would tolerate (BB 39.8/40).
But Britain and Ireland were unified.
The Wars of the Roses
These covered most of the period (1455-1485). A period of dreadful poverty. For almost all the period, annual military maintenance exceeded annual census taxes. But minting was kept to an absolute minimum, very few investments were made, and with the help of an exceptional year, inflation rose no higher than 3%.
In 1485 Henry VII rose to the throne and instituted a set of reforms. Together these acted to reduce (a little) the risk of revolt, radically improve domestic policy (23 DP clicks, in total) and improve tax income. In 1486 income was 66% higher than in 1484.
We now find ourselves economically under-developed and behind the world technologically, BUT WE ARE ONE NATION!