And a good question. I saw the blue tick and the words 'Official page for #BlackLivesMatterUK' in the bio and assumed some kind of official status. But I think you're asking what gives these people the right to make that claim and why Twitter has deemed them worth of the blue tick and I have no idea to be honest.
It's all a bit shrouded as to who is behind it now (The UK one)
It seemed to be formed back in 2016 and first came to prominence with the
London City Airport closureThis led to
charges of those involved (at that time).
They were found
guiltyFollowing the blockade it emerged that several of those arrested were veterans of climate change protests and their involvement at City airport led to accusations that the British offshoot of the Black Lives Matter movement has been hijacked by middle-class white activists.There are photos of them there, some undoubtedly likely trust fund kids.
A quick google on their names reveals several seem to be still active in a
give me a cause and I will agitate sort of way, CV's stating writing for Novara and such.
There is also a reference in this book
here that anti racist campaigners accused them at the time of being "white left wing activists hijacking the Black Lives Matter Banner" though I can't find who that was.
I have no idea if any of these, or their fellow anarcho left political travellers are still involved, but it's always a bit of a problem when these sort of professional agitators find a noble cause to latch onto and then try to twist it to their own ends.