I do not see "Brit" as an insult myself

It frequently is, like most such words.   It depends who's using it and why. After all "Yid" is just the Yiddish word for Jew.

One syllable abbreviations for nationalities tend to be particularly prone to be used as insults. That's why "Jap" sounds more offensive than "Japanese".
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Did you really think, Jack, when the blokes in Top Hats stormed in and ripped up the landscape, that the message you were being sent was an unambiguous "What a lovely thing to happen, how proud we should all feel about it"?

I suppose that's one of the multifarious ways of reading it that Danny Boyle was talking about. Let a hundred flowers bloom.