On the America Side:
Douglass MacArthur.
Joking aside, I've always liked "High Chancellor" or (if that's a bit much then just) "Chancellor".
There is, of course, the possibility of "General", "Marshall", "Grand Marshall", or a similar military style title.
You have mentioned President, which is very possible, in my honest opinion.
Then again, depending on the man, he might go ahead and opt outright for "Emperor". "Consul" is another option, or varients of it.
On the British Side:
"Lord Protector" I would say works very well, as would previously mentioned military style titles.
I suppose "(High) Chancellor" could also work, but IIRC, there are actually already Chancellors or sorts in British Government (the one coming to mind being --- of the Exchequer).
I personally think "Emperor" would be a bit less likely in Britain, particularly if the monarchy is kept (sort of) in tact a la Spain.
"Magistrate" I suppose is possible in the UK.