I have never has a British special forces member tell me that they have been in one of the units and it is only after knowing the guys for a considerable number of years have I had my suspicions the guys are ex SAS/SBS but even so I have never been told.
I even worked on a project that was clearly for the SF and all the guys just said they were attached to the MoD.
When someone tells me that they have been part of the special forces I take the view that it is highly unlikely they are as they say .