I believe that the london flights of Air India and Emirates will be using Birmingham, BA and Virgin will no doubt have plans in place to use alternate non BAA airports for their longhaul schedule
It is one thing for an airline with 3 or 4 flights a day to up sticks to another airport, it is something totally different for companies like BA and VS. BA have c.120 long haul aircraft with probably 70 movement pairs per day. Give everyone an idea where exactly you think they could move any sizeable part of that operation to?
Then you have to consider the crewing implications re duty hours operating from a different base, hotel requirements for crew and pax, bussing etc etc.
I would suggest that BA (and VS) would just shut down the operation for a day or two. The ramifications of that in itself are substantial and will take about 2-3 weeks to work themselves out of the operation, if past experiences are anything to go by.