I've flown Manchester to Heathrow before as part of a longer journey where I can't fly direct to my destination.
The reason has been because of flight times, cost of trains and cost of hotels.
I live within 25mins drive of Manchester Airport.
It's not cost effective nor time efficient to drive to Heathrow. Its about 4hrs not allowing for traffic. If you have an early flight, you either have to drive through the night or get an overnight hotel at the airport plus parking.
So that's petrol cost, plus hotel cost, plus car parking for the duration of your trip. And potentially an extra day off work at one or both ends of your trip, depending on flight times.
Or you can get the train. Train is 25 mins away (plus waiting time) and then just over two hours to central London, and then you have to change to get to Heathrow. Realistically the peak hour fares mean that its prohibitive to travel at certain times. Or its just a none starter if you have an early flight out of Heathrow. So again you are looking at an overnight in London.
Your cost then becomes your train fare, plus hotel.
Or you can pay maybe an extra £50 per person to go via Manchester fly and change in Heathrow. For the cost and convenience it's a no brainer.
If they put landing fees up at Heathrow, if they didn't at Charles De Gaul for example, all you'd do would be to fly to Paris instead of London as its really not much difference in time and then on to your destination from there.
So unless you had all the airports close to the UK doing the same thing and all putting up fees, it won't change habits of travellers from Manchester travelling via Heathrow.
Ultimately the question is why would I travel by train or car to Heathrow rather than fly from Manchester wherever I'm headed because of the travel times and cost? A more joined up airport plan for the North would make more sense than putting up landing fees by a huge amount. I'd be willing to fly out of Liverpool, Manchester, Leeds or Birmingham in theory if I had to if it was cost effective because travel times mean I don't need the additional hotel in the same way. Plus I much prefer flying through regional airports because the experience is much easier and less stressful than the hell which is Heathrow.