Durham is about 15-20 minutes south of Newcastle, so you can subtract that from the journey time from London. Do be aware that the direct journey time Newcastle-London varies between 2:50 and 3:25 depending on which train you take.

You won't get TPs on the cheap BA flights, but you will get some miles. Although at 125, it's hardly worth getting excited about.

You could also look at easyJet if you want to save cash. It can be as little as £14 including tax each way, although you will probably pay as much again to schlepp out to Stansted. Again, the southbound journey is a lot smoother than the northbound one.