As with Heathrow, most pax were interliners, my guess would be 90% minimum. So competition, wasnt with the railways as previous posters have suggested, as pax were using the service for onward travel around the globe.
galaxy68 - I can certainly believe that a substantial part of the traffic from Leeds to Heathrow would have been onward connections. A figure of 90% seems a little high, but in the absence of any figures of my own, I can't claim otherwise.
However...
A single slot pair has in recent years become worth a very large sum of money. An airline always has the option to sell the slot to another airline and just put the money it gets on deposit in the bank, so we can consider the value of the slot pair as a sort of investment - not dissimiliar to owning outright a brand new 737 (i.e. no interest or lease payments involved).
An airline can choose to use that LHR slot pair on a low yielding route, but may well believe it can get a much higher return on the value of its LHR slot by flying a different route instead. bmi is thus choosing between a LHR-LBA route or a LHR-KBP route which would probably bring in much more profit per slot.
Yes, Leeds, Durham, Belfast, Liverpool, Inverness, and plenty of other places in the UK regions need and deserve to get direct connections to LHR for the good of the country's economy, but for this to be economically worthwhile to airlines, LHR needs a drastic increase in capacity, which means a 3rd runway. Further, unless bmi merge with Virgin, the only carrier for whom flights from LHR to the regions would be sufficiently profitable is probably BA.