"Dictator" implies illegitimacy, unpredictability, or spontaneity -- coming to power by some sort of coup or political surprise. More importantly, it implies being entirely unfettered by law, custom, and the wills of one's subjects -- like the Roman dictators were -- which isn't even true of Cromwell, and certainly isn't true of the kings.
You'll find in most definitions, people go out of their way to not lump Kings in with dictators. Because Kings are the dictators we like for some weird reason. And it usually leads to bizarre contortions.
Even with your very specific definition, it pretty much applies to a whole bunch of kings.
edit: hah, found a blatant example from dictionary.com
Dictator: a person exercising absolute power, especially a ruler who has absolute, unrestricted control in a government without hereditary succession.
How weird that you escape the title "dictator" just by dictating that your kids will be the dictator after you!
But we do love a bit of heriditary military dictatorship here in the UK.