You might have second thoughts about that last sentence if you were to look up the ratio of terrorist plots detected and prevented against attacks actually carried out.
Too many attacks have come from “known” i.e. watchlist lone wolves. The British have a reluctance to due a deep surveillance on Moslems. The mayor of London is a Moslem anti-Semite and Marxist. He controls the regular police to some extent.
MI 6 would be out of his purview. Probably the same for MI5 but they have been infiltrated enough to raise eyebrows and worries.