While I've never tried this particular one myself, I have cleared security at B3 to use the Lufthansa lounge when flying United out of C, so in theory it could work. You will indeed have to clear security twice, and I don't recall there being a pre-check line at B2 if you're a member (there definitely is at B1). Be ready, however, for the Virgin lounge agents to be confused when you arrive. While the rules permit you access (business class on Delta or Virgin between the US and UK) they probably haven't seen many Delta passengers trying to enter their lounge.