What took so long? That was our first thought when we heard that a TV crew spent three days trailing Sandy Poirier, owner of the trendy South Boston hair salon Shag. Poirier says a crew from London-based Lion Television – they produce “Cash Cab,’’ among other shows – are putting together a pilot about the salon and its eccentric owner. “They like the hot Shag stylists and me, the rock n’ roll badass motorcycle guy,’’ says Poirier. (Apparently, there’s interest from the A&E and Oxygen networks.) Poirier, who’s heavily pierced and tattooed, told us the crew shot him at the salon, at home with his dog, and out on the town.