However, hubbing from abroad via London is becoming rarer these days due to APD - many people transitting through European airports find it cheaper to do so via Frankfurt, Schiphol or even Istanbul.
Sorry to detract from bmi Regional, but you don't pay APD on connections. If you're on a through ticket, APD is charged from your first airport of departure to your final destination. So, wherever you transit, you pay UK APD for the whole journey you undertake. If you're on separate tickets, then it's different.