On return out of LHR, VS (it's VS, not VA) and DL operate a joint operation for most part. As D1 passengers you'll have access to same lounge as VS UC passengers. I'm not sure if spa services are even being offered again. You'll use the same special drop off at LHR regardless of carrier. (Make sure to tell car/taxi to drop you at the Virgin Upperclass checkin, not at main terminal)

As for the onboard "lounge", you're not missing anything. In all my flights I've seen it used once by 2 people. All it is, a bar chair,, and some self pour drinks..



VS "onboard lounge"


Lastly, the VS 787 UC seat pales in comparison to DL seats on all but the DL 767-300. The VS A350 doesn't fly most routes, so likely were booked on the 787.