Britain, Briton

Britain ( always capitalized ) means England, or, more specifically, the island that contains England, Scotland, and Wales--
England, Scotland, and Wales are called Great Britain.
Arthur was once the king of Britain.

Briton ( always capitalized ) means a British subject; someone who lives in or who comes from England--
Yes, a Briton is a foreigner in the United States, although we speak the same language.
You can tell by his accent that he's a Briton.

Now that you know that, you can say--
"He's a Briton, and sometimes gets homesick for Britain."

Note--"Britons" is the plural ( more than one ) of "Briton", meaning a British subject; someone who lives in or comes from England--
They are Britons; that's why they sound strange.
Note # 2--"Britons" may also mean one of the ancient tribes that once inhabited what is now England--
England was inhabited by several different tribes of people--the Angles, Britons, Picts, and Jutes were there when the Romans came.
Note # 3--"Briton's" is the possessive ( belonging to ) form of "Briton", meaning a Britsh subject or an inhabitant of England--
He has a Canadian's manners, but a Briton's accent; he has lived in both countries.
"Briton's" may also be a contraction of "Briton is"--
This Briton's too homesick to stay much longer.
The possessive form of "Britons" ( more than one ) is "Britons'  "--
The Britons' territory now includes all of England, Wales, Scotland and Northern Ireland.
Note # 4--"Britain's" is the possessive ( belonging to ) form of "Britain", meaning England--
Elizabeth is Britain's queen.
"Britain's " may also be a contraction of "Britain is "--
Britain's going to discuss that with the United States ambassador.