There are a bit over 3 million passengers from Ireland, North and South, to Heathrow every year. While BMI are probably responsible for something in the region of a million of them, that still eaves around 2 million for Aer Lingus.
In this context 150K sectors isn't that much at all.
And the proportion of connecting passengers is likely to be lower on DUB-LHR than the other routes considering Dublin has connections to many hubs (Paris, Amsterdam, Frankfurt, New York, Dubai) that are scarcely there at the other Irish airports.