Here's why UK is prone to terrorist attacks

Intelligence officers have identified 23,000 jihadist extremists living in Britain as potential terrorist attackers, it emerged yesterday. The scale of the challenge facing the police and security services was disclosed by Whitehall sources after criticism that multiple opportunities to stop the Manchester bomber had been missed.
About 3,000 people from the total group are judged to pose a threat and are under investigation or active monitoring in 500 operations being run by police and intelligence services. The 20,000 others have featured in previous inquiries and are categorised as posing a “residual risk”.