This all mainly of Virgin's making, not BA.
Virgin had the chance to do a tie-up with bmi, both before and after Lufthansa bought it, but they let it fall into the hands of IAG. Virgin's fleet decisions, failure to join an alliance are all problems of its own making.
Where BA is hitting Virgin hard is with the transatlantic venture with AA (and the amount of money it's now investing in the brand) but this didn't happen announced and Virgin had plenty of time to anticipate and respond.