The ticket I had was valid for through check in, as later confirmed by both BMI and Virgin. I fly a lot on business and specifically discussed this point and had confirmation from Virgin themselves (with whom I made the booking direct), prior to flying.
For the record, on the return journey, checking through at Virgin check in in the USA, I got the same crap, and the situation was only remedied when I insisted I speak to a manager. I was of course correct.
It seems the check in staff for both BMI and Virgin, assume the ticket will not be valid for through checking, without even looking at the ticket.
Incidentally, if you are flying on from LHR after an overnight flight from West coast USA, ensuring bags are checked through is very important. I do not take the risk, and fly BA because of this.