The whole "terrorist" threat is nonsense. It's just scaremongering by the Government that wants to appear like it's doing something, and it also keeps the population fearful and easy to control.
Let's not forget you can drive an articulated lorry (full of anything you like) around Central London and up to the door of any major supermarket in the UK with virtually no checks/controls.
RAND report: Threat of homegrown jihadism heavily exaggerated, Zero U.S. civilians killed since 9/11 | loonwatch.com