The problem Virgin and the fare paying public have, is that if BA get too strong, they could eventually have a long term goal of pushing Virgin out of LHR, regardless of how good their product is, by purposely offering fares on the Virgin route structure at a loss. Eventually having a total monopoly on short and long haul out of LHR and, therefore, charging what they like.
I call BS on that.
Not only is LHR bigger than BA, but anti-competition stunts like that would not go unchallenged and legal proceedings would ensue.
The problem with underpricing is, often, there is evidence of it. All you have to do is look at tickets prices in the past to raise suspicions.
My own opinion on beardy going nuts about this IAG/BD deal is that he wanted the LHR slots and now that it looks like he won't get them, he's throwing the toys out of the pram.
If he was genuinely worried about British aviation, jobs, the travelling public, etc then he should have tendered a serious bid. Instead he only put down half the amount of the IAG bid on the table. VS are a niche carrier long haul carrier with no domestic network or experience with an interest to expand LHR operations. There is little chance beardy wanted BD to run it.