[Prophet in the wilderness]

" It also happened just as the leftist allies of the Muslim extremists staged large protests in Scotland, requiring thousands of police to be transferred there rather than protecting London " .......
Maybe the leftist allies of the Muslims were a part of the planned attacks.
It does not look like a coincidence to me that the protesters had the police away from London at the SAME time that this attack happened.
If I were Scotland Yards, I would start investigating those protesters ( all of them ) and see if any or all of them had any contacts with the terrorist.

[Wallace T.]

If I were Scotland Yards, I would start investigating those protesters ( all of them ) and see if any or all of them had any contacts with the terrorist.

The radical left and Muslim extremists have been in an alliance for decades. Despite the Soviet invasion of Afghanistan in the 1980s, which brought Communism and radical Islam into conflict, both groups generally see the democratic, capitalist West as their primary enemy.