On the term 'Brit' as a term of abuse,

but now, characteristically, a smugly favourite term
used by the English in reference to themselves :

I first heard the word used contemptuously
by Irish Republicans in Belfast in 1970.
It was probably originally imported
during the Irish liberation movement
from the U.S.A.,
where it was apparently first recorded in 1901, and
was considered offensive by Englishmen travelling in the States, who regarded it as another instance of the "odious vulgarism" of the Americans. 
But Bret and Bryt were common Old English words for the (Celtic) Britons and survived until c. 1300. In Old French, Bret as an adjective meant "British, Breton; cunning, crafty; simple-minded, stupid." [from the online Etymological Dictionary]

* * *

In recent years, unfortunately, the word Britain 
has largely been replaced by the awkward and ugly U.K.
– even as far as changing the international vehicle plate
(after Brexit) from GB to UK. But an F can easily be added.