It takes about 30-35 people to run 24/7 surveillance on one individual. I don't know exact figures for persons of interest to the security Service, but it is certainly in the thousands. Cuts or no cuts, it's simply not possible to monitor all of these people all of the time so it becomes essentially a matter of risk assessment. Occasionally they get it wrong, and someone deemed not to be an immediate threat does something terrible. These odd balls are often 'all mouth' and it is often difficult to predict which ones will become the next headline. Radicalisation is broadly understood ,but the last couple of weeks (often all it is) before an attack where an individual goes from being just one of thousands of loons to a killer is not that well understood from what I can tell.