Are Intelligence Agencies' Hiring Procedures Too Strict?

[Stan]

Didn't the news report that MI5 claimed that Scotland Yard and ISI were mobbed-up by jihadists? ...

I think you may have this somewhat backwards. The MET (SO13/15 Counter Terrorist Command) not only claimed, but also investigated and concluded the existence of Jihads in/and from ISI, MI5 and 6. The enormous amount of intel and forensic evidence that was initially ignored, is downright scary.

I will once again ask you to go here and introduce yourself.

Leaving aside history for the moment, isn't the problem insiders change their loyalties rather than infiltration?...

David, I agree, an inside job. On track with this thread, the factors that disqualify candidates for high-level clearances are validated by well-documented track records both in the US and UK. On the other hand, even when we do discover an internal problem, someone decides the potential benefits outweigh the inevitable disaster. I recall several 1999 and 2001 MET case studies that fell upon deaf ears.