IAG/BA will never buy Virgin Atlantic. Sir Richard Branson would never sell to BA and Willie Walsh wouldn't want to give him the money.
BA buying Virgin would be a futile endeavour because the European Commission assesses the impact of consolidation on a route by route basis, and not by reference to slots held at airports. So, for nearly every overlapping route (which is almost every Virgin route) BA would have to divest of a slot pair to a willing entrant, essentially rendering a merger pointless.