Was King Arthur influential in Medieval England... How?

King Arthur did not live during the Medieval Period - though Hollywood always dresses him in the armour of the twelfth century - so as a person he was not important.

As a person, it is possible that King Arthur never existed at all: he may well have been just several groups of legends that got bound together into a single story (which is probably what happened with Robin Hood). If a real Arthur ever did exist, it was probably during the Sixth Century or thereabouts - a long time before the middle ages.

But as an idea, Arthur was very important. Arthur was a king of the Brythons (the Welsh) who had ruled over the whole of Britain. Any Welsh king who could claim descent from Arthur could call himself the rightful king of all of Britain - and any English king who descended from Arthur could also claim to be the true king of the Welsh.

The idea became important when Henry of Richmond landed with an invasion army at Milford Haven in 1485. Years of civil war (the Wars of the Roses) had killed off most of the best claimants to the throne of England, so that the throne was occupied by Richard III (who had a very poor claim) but was being contested by Henry Tudor (who had even less of one).

But Henry Tudor was Welsh on his father's side. He had the brilliant idea of claiming that the Tudors were descended from King Arthur, and that this made him the rightful king of both Welsh and English. Since the Welsh and the English had been at war almost constantly for almost six reigns, both sides would have been glad of a king who could claim legitimacy for them both. When Henry defeated and killed king Richard at the Battle of Bosworth he immediately began working on establishing his descent from Arthur (which made the Tudors the rightful kings of both nations).

Henry's son - Henry VIII - unified England and Wales into a single country, and his daughter - Elizabeth I - helped the Welsh translate the Bible into their own language. Many prominent Welshmen worked for the new kings of England (Elizabeth's spymaster Lord Cecil, was Welsh), and the longstanding enmity between the two countries rapidly faded.

So King Arthur wasn't important as a person - perhaps he never really was a person at all. But he was important as an idea. One might say the same thing about Johnny Appleseed, Uncle Sam, Hereward the Wake - and perhaps even some real people. Abraham Lincoln and Winston Churchill were both real people, but their myths may be even more important than they were.