VS did manage a shift to LHR and survived and prospered, if it had been confined to LGW and MAN without access to LHR, history would probably not have been any different.
Branson does state in his aurobiography that VS would have gone under if they had not gotten into LHR when they did in 1991 as the first Gulf War had just broken out and flying took a major hit.
Long haul is about hub connectivity and LHR has a huge historic advantage over Gatters. Any attempt to build MAN into a similar hub would face a the same issues sadly. That's noto say VS cannot now make the most of the Delta connection at the US end to grow out of MAN, and not before time IMHO.