Any advance on early 15th c?
Excellent! A myth debunked - thanks to Ursa and Yer Honour!
I will henceforth expunge this particular objection to James I.
So I presume the same applies for guv/Guv?
Yes, but even more so. "Guv" is short for "governor", which is no longer a title in such wide circulation as once it was. You might capitalise the Governing Board of a school, but for coppers, it's far less formal. In fact, I've never heard it used by coppers outside The Sweeney,
although it is more of a southern phrase than a northern one, so they might still use it Dahn Saaf. Up here, they,d say "sarge" or even "boss", but for the most part, it's nicknames*.
So, as it is an informal, almost slang mode of address, I'd part company with Her Honour and always spell it "guv". Same with the following regional methods of address:-
"Now then, squire." ("Hello" in demotic Lincolnshire).
"Alreet, lad?" ("Hello" in demotic Yorkshire).
"Awreet, marra? ("Hello" in demotic West Cumbrian).
"Areet, chaw?" ("Hello" in demotic North Cumbrian when addressing a young or middle aged man. For old men, subsitute "gadgie").
* In common with established British nicknaming convention, all formed by chopping off any loose consonants at the end of the name and replacing with *y.