I don't know what the lake was like where you were, but wind conditions were 12-knots—ideal for sailing—until about 10-AM. Winds suddenly slackened about the time I was supposed to quit for the day to meet an appointment.
At that time, I could see no other boat on The Broads! 
Many are unfamiliar with sailboat operation and terminology, so your complaint only makes sense to a few.
(Even then, important details were left out—early/late...winds...sailboat under power...markers...speed...gusts...depth...windsurf er...GPS...et cetera...). "Adjusting your course" suggests the use of GPS—at a time when "there were miles of water to play in").
Unless you are actually on the sailboat you've complained about, there can be several reasons for his actions that would appear "unaccountable": One especially being,
don't expect him to be relying on a strict GPS course to ply these waters.
The reasons are many for allowing a sailboat an excess of "elbow-room", especially with "miles of water to play in".
A "starboard tack" suggests the sailboat's skipper quite possibly never detected the direct approach of your boat. (If you could plainly see the skipper, he would have been on a "port tack").
Besides, tacking would be accomplished by a slow-motion turn to pick up "new" wind: If I'm reading you right, his turn of 180° would suggest an exaggerated response to your unanticipated approach. (Or possibly an emergency response to an abrupt change of wind direction).
Nearly always being
the privileged vessel means it's possible this skipper was lulled into having a satisfying and carefree cruise:
A loud signal would have indicated your intention to pass one way or the other and to give him time—
at headway speed, incidentally—to anticipate your speedy approach and—the
as-yet-undetermined intent of your actions.
Although I was not present—and you were unarguably "the burdened vessel"—one or two toots of the horn would have been the appropriate alert in this circumstance.
IMHO.