Hehehe ;P
騎士王, which literally translate into the "King of Knights", but the fact that 王 is just basically refer to the "Rightful leader", it does not necessary to be male, but because the fact that there are rarely any female leaders back in historical time, hence which is often to be translated to be the King. Generally it is being referred to as the male leader as there is pretty much only male leaders back then, but it should really just be the "Leader of the Knights".
Yes, you're right that she's BASED on King Arthur, but she isn't King Arthur, hence the name change, King Arturia.