If your routing ends up as PNS-NYC-LHR. you should familiarize yourself with the extra benefits that PMs have (such as,
access to the Virgin Clubhouse departure lounges). NYC-LHR in C+ on DL763/4 is fine if you can score a window/aisle combo. Premium Economy on VS is even nicer if you can find a reasonable fare.
VS offers a bid-to-upgrade program but only for flights ticketed on their stock. Otherwise, they offer day of departure upgrade$ at the airport.
DL Plat on VS metal (code-share) how to upgrade?