So, the battle of Lutterworth is over, and with it - the campaign. The Yorkists rout the Lancastrian army of Somerset and capture the rather confused king Henry, complete with his unfortunate arrow-wound in the neck, and York explains to the king just why he was compelled to take up arms to drive away his evil councillors, thus saving the realm - or at least, that's the official version. The battle of Lutterworth scales out as having about 1900 casualties, but as this is the first resort to arms it's likely the battle's death-toll would have been lower as many troops fled rather than fight to the death. (The historical equivalent of 1st St Albans, for example, had only around 50 to 300 dead and was more of a massive gang-rumble in the town's streets than a battle, which generally only revolved around the deaths of targeted Lancastrian commanders.)
None of the Yorkist commanders died, which is at least historical. Of the Lancastrians however we have the lords Devon, Northumberland & Egremont killed in the fighting. This is okay in game-terms by and large, as Egremont would be historically killed at Northampton in 1460 and merely exits a campaign or two earlier. Northumberland died historically, and while Devon survived the historic campaign he would die naturally in 1458 before the next outbreak of fighting. Clifford is captured by the Yorkists, but (since he was one of the very few targetted die-hards that were specifically killed at St Albans) he is executed as a traitor by York.
Somerset was historically (and famously) killed at St Albans, but here he routed from Lutterworth and successfully evaded capture. He has to flee the country to escape belated capture and execution – as the battle of Lutterworth is further north than the historic St Albans, he has time enough to flee to London and take a fast ship to exile, most probably in the Lancastrian bolt-hole of Scotland. His survival means the scenario was only an intermediate Yorkist victory. How Richard of York must have regretted his hated rival giving him the slip!
The Earl of Wiltshire and the Duke of Buckingham are both linked by marriage to Somerset, but were historically pardoned by York and so the same happens here. Buckingham especially was a conciliatory figure and so he is able to remain in London to see the city entered by the Yorkist army & King Henry VI. Wiltshire was York's one-time rival for the lieutenancy of Ireland, so he presumably keeps a far lower profile for safety.
A July parliament pardons all the Yorkists for Lutterworth, blames the absent Somerset for it and in November King Henry falls ill once again. York is proclaimed Protector and Buckingham works to help run the country, hopefully overcoming the one-off day of violence. Warwick gets rewarded with the plum post of the Calais captaincy. It's all great news for the Yorkists - what could possibly go wrong?