The NA - UK non-stop flight requirement for access to the VS Clubhouse applies to all of the above categories (D1, Upper Class, and Gold+ passengers). You are not entitled to access with just a domestic D1 flight, or any other D1 itins that do not include a NA - UK non-stop. Also, a Gold+ in not entitled to access on any international itin as VS is not a Skyteam member.
I can't tell if I'm reading this correctly (I think I am). I have a LHR-ATL-FLL route in Delta One. Because of that alone I should be able to use the Virgin Clubhouse in Heathrow before the outbound flight even without status with Delta?