By definition, in their line of work if you knew exactly how useful and effective they were they would not be doing their job properly. They report to the UK government, not to you and not to me even though I am a UK citizen (as you may be, I don't know). They have worked for governments lead by or including all three main political parties in the UK and they all decided they were useful enough to them to keep, in pursuing their goals on behalf of the people who elected them. That's good enough for me.