Off the top of my head, Washington, Lincoln, Grant, Teddy Roosevelt, Truman, probably Eisenhower, and Kennedy. At the time, the were unit commanders rather than President. Which is why the PM is indeed the relevant comparison -- a purely ceremonial status as "head of state" doesn't matter, the fact of being elected at some point in life rather than born to it does. Also the shortened timeframe makes them a better match, since with US Presidents we're talking about more or less modern warfare.
I thought Washington and Lincoln were Englishmen? ;-)
I take your point, but mine still stands - the Royal family send their sons to battle, they do not protect them from the harm that may befall any good englishman defending his country. The case is not so clear cut with the sons of US presidents.