Black people make up 3% of the UK population according to a quick Google search. If there are 7 black people in his company of ~250, that's almost 3%. This is just one data point but it's the one he's using in his post and getting upset over the lack of black representation, when it's proportionate with the overall ratio of ethnicities in the wider population.
The 44% figure comes from the Greater London Authority, but should be clarified a bit. There is a BBC article[0] about this which states "... 44% of the city's people are now of black or ethnic minority origins".
I don't know if London demographics are the point of comparison, e.g. if you looked at FAANG's demographics you would not compare them to the Bay Area as their hiring pool is much wider than people who grew up locally.
