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.
Now that the pandemic has made us all armchair experts in mortality statistics, it's possible to get some perspective on the scale of Islamic terrorism in the UK. The total number killed is 95. This is about 0.3% of the 3000+ killed by IRA and other Irish terrorism in the 1970s/90s, about 1.5% of those killed by accidental drowning, and about 3 times those killed by lightning strikes.
I mention this only because the impression is sometimes given here that Islamic terrorism is virtually a weekly occurrence in the UK. Of course one death is too many.