I suspect that the fairly recent revelations by an ex-SAS chap who trained the armed police have some relevance. He described many of them as far too gung ho and psychologically and physically unfit for the job. He also pointed out that the SAS were not allowed to fail them.
It might also be relevant that an earlier interception was not made apparently because the armed response unit that these cowboys belonged to was not yet in place to screw things up.