If you want to fly in a 747, using Delta miles, Virgin is about your last bet.
DL is getting rid of its remaining 747s n a few months, AF got rid of theirs, KLM has some but using miles is basically impossible. AZ and SU don't have 747s.
Virgin got rid of 747s flying out of LHR a few years ago. They still have 747s flying out of LGW to leisure destinations (LAS, MCO, Caribbean, etc.). Because of limited premium demand, the upper deck is a mix of E/PE, and only the nose cone is J.
On my last few vacations involving travel to Europe, I managed to fly back to the US on a LGW-leisure city flight. Enjoy it while it lasts.
Virgin's lounges are very nice.