It’s starting to feel like the Iranian government needs a boogieman to blame because brutal force isn’t working and with the US having no official diplomatic ties much less an embassy, they are turning to the British. It probably also infuriated the Supreme Leader et al, that the BBC was actually reporting their atrocities for all the world to see.
This puts the UK and her allies in a pickle. Technically this isn’t an outright act of war so long as those arrested did not have diplomatic immunity, which is a courtesy anyway, and they did not make the arrests on British soil. However; it can clearly be seen as a sign of aggression.
It also reads much like the government is baiting outside countries to get involved to lend credence to their shrill assertion that the US and Europe are the real men behind the curtains.