I think this is answered in the opening scenes, when William pretends to be Sir Ector. They make a point that the other knight would need 3 more points to win, which is impossible, so he has to unhorse William if he wants to win.
And a robot, and a damn good spaceship pilot and managed the Phillies, truly a renaissance man.
Of all the baffling things in that scene, most seem to have been adequately explained here. But I’m still trying to figure you why you’d yell your own name in this situation.
He had just been knighted under his true identity, so it would make sense if he had yelled ‘I… AM… WILLIAAAAAAAAAM!’ or something like that, but the name on its own is weird.
Bending over backwards to accommodate the idea there’s a historical truth behind this, I believe that knights/nobles - and their retainers, men at arms etc - did use their surnames or titles as warcries “A Percy”, “A Warwick” for example. And c. 50 years after this we had “Cry God for Harry!” (citation needed).
But yeah, just the first name is a bit weird. Is the idea he didn’t have a surname? Too confusing anyhow - it feels like 90% of the nobles of this time were called John, Henry, Richard or Edward so not brilliant as a friend-or-foe identification scheme.
"Oh, you meant Harry Percy! No, we’re Harry Plantagenet mate. Yeah, no, easily done. Our fault, if I’m honest. Me and the lads will have a right laugh about this later I tell you.
Kill him."
He did, it was Thatcher. An important plot point, because it establishes that he’s ultimately responsible for Margaret Thatcher ruining the United Kingdom.
Child of a small time businessman, seizes limelight when the old guard can no longer deliver, wants to liberalise hidebound tradition, achieves support in the City of London by unleashing a Big Bang… I can see it.