Why this movie is britishness ?

this serie is british principaly because of the city where it takes place, London. it's the capital city of England, so, of course, this serie is british. But also because characters are british, the "vo" is british, a scene in the serie takes place on the tower bridge, and i forget some points for sure. So it's all of this littles details that make the serie litteraly british. And for finish,this serie was Based on the Penny Bell investigation in 1991.