Unless Virgin can get Manchester-London/Heathrow T5 then there is no point. Over 50% of BA passengers travelling with the airline on the route make connections through LHR and LGW to destinations across the globe. Same with VLM, who offer the business passenger the perfect way into the heart of London. No hassle! Both airlines have proved this and I can only see T3 or BD removing the routes (which I find won't happen when BMI launch TATL in 2009).
Also, launching long haul routes from the airport is fine and dandy, but most travellers on the connections flights choose to fly BA due to their high quality standards.
Personally, I would rather fly MAN-LHR-YYZ with leg room and my own IFE instead of sitting cramped on a MAN-YYZ with TCX/TS/ZM/GSM with a worse service and only my magasine for company and I'm sure many of the 800,000 connecting passengers think the same. I find LHR a great airport to pass through and would choose it any day over AMS, CDG and FRA as it is home to the best airline in the Western world - BA whose service is great!
P.S. I do not work for BA or have any beneficial connection with the airline.