The Security Services have no failed in this instance, and the BBC can f**k off pushing this agenda.
The best the security Services could have been expected to have done in this instance is carry out a periodic check on this individual. In truth, probably just a quick PNC check to include known associates. If anyone of his known associates pops up on a list of people of interest then they would do some digging. Other than that, they wouldn't have put him in a high risk category.
There are hundreds if not thousands of people who fall into this category, and any one of them are capable of carrying out an act of terror; either alone or as part of a group. MI5 simply do not have the resources to watch every single one of then 24 hours a day.
This guy has fallen off their radar because he kept himself to himself for the most part, and the only reason people are making connections now is because he actually went and did what he did.