I don't think so. He only did a couple of years at 21 (TA) SAS before breaking his back, had no rank and certainly wouldn't have been an instructor. Mears actually has instructed combat survival on UKSF selection.
Rank doesn't necessarily mean he didn't instruct, he was most likely the troop specialist in that area, he was also a combat medic and various other things.
And although it was TA SAS, they still had to pass selection which he gets my respect for.