Decolonisation is complicated in Britain because we're still not one nation: we're Scottish, Welsh, and English (and that's without trying to account for the many Brits not born here). So there's anti-English decolonisation, and class decolonisation (a long-term project for the English beginning in the 1300s but excelerating in the 1900s), and incomers from ex-colonies, and that mix has all been happening simultaneously. But Brits have decolonised before: after the Roman Empire, when England finally assimilated the conquering Normans, when Protestantism replaced the Catholic Church, &c.

But, simply, yes, RP is still an issue (although many posh boys from Eton are wanna-be black hip-hopsters o_O ) although less of an issue than when I was a kid, and less for me than for my parents and grandparents (who could've been beaten by their teachers for speaking home-language in their village school).