From the CAA report linked above:
3.3 Much of the growth in connecting traffic can be attributed to the rising number of international - international passengers which has quadrupled from 4.4 million in 1987 to 17.9 million in 2007, whereas total passengers at Heathrow have only doubled over this period. The increase in long haul services at the airport has given rise to greater numbers of domestic - long haul connectors (1.5 million in 1987 to 4.0 million in 2007), short haul - long haul connectors (3.1 million to 13.1 million) and long haul - long haul connectors (0.7 million to 4.2 million).
I'd trust the CAA data, although somewhat dated, over an airline that doesn't fly to Heathrow.