That doesn't say that he was in the SAS. It just has his name and SAS in the same paragraph.

It says that in the past MI6 needed a person with military skills do something, as was the case with Crabb, so they decided to start cross-training SAS people to do that in the future.

Neither the Wikipedia article nor this endnote says that Crabb was SAS.

I was just hazarding a guess at the question: "Where did you get the SAS part from?"