If services exist from Heathrow to A and to B, how on earth will you prevent or dissuade passengers travelling from A to B from connecting over LHR?
Well for a start how about making transfer passengers pay APD tax so they actually contribute something to the UK economy rather than just the bottom line of a few network airlines ?
At the moment APD is a much bigger burden on point to point passengers who actually want to come and spend their money in the UK, and on UK citizens who want to travel out of UK airports.