I suspect the reason for Virgin being late to the party is because of securing financing. No one knows just how solid the financing is, bearing in mind due diligence has not completed. Bearing in mind the VS offer is reported to be 50% of the IAG bid suggests it isn't backed with unlimited cash
LH is of course happy to entertain an offer from VS as it strengthens its negotiating position with IAG.
Due diligence is expensive but even if Virgin aren't successful they will acquire knowledge of bmi's business and markets which will have some ongoing value to the business.