I think this answers most of your questions:

"Virgin Atlantic Clubhouse lounge access is available to BusinessElite® or Upper Class passengers and Diamond or Platinum Medallion® members on same-day travel when flying on a Virgin Atlantic- or Delta-operated transatlantic flight to or from the U.K. Plus, Diamond and Platinum Medallion members are allowed to bring one guest. Clubhouses are located at London Heathrow, London Gatwick, New York-JFK, Newark, Washington, Boston and San Francisco airports. Please travel with and be prepared to show your Medallion card to the Virgin Atlantic representative to receive Clubhouse lounge access."

Not mentioned specifically but (I think) mentioned earlier in this thread:

Arrivals lounge access: Yes.