The demonstrators, if such they still be, on the streets of Belfast have no more nor less British a culture than, in order of their existence within the present boundaries of the United Kingdom, the Quakers, or the Sephardic Jews, or the Hebridean Catholics whose Islands the Reformation simply never reached, or the native speakers of the language that is now called Welsh.
But neither the Quakers, nor the Sephardic Jews, nor the Hebridean Catholics, nor the native speakers of Welsh, imagine that everyone else in the United Kingdom is like them, or at the very least that they sit well within the British cultural mainstream. Nor do either the Friends or the Sephardim imagine that such has ever been the case. Ever.
Therein lies the difference.