If I need to be at Heathrow for that time, I make sure I am there not much after 7.00am and use the time relaxing with a coffee / breakfast / shopping. (I reckon I could claim shares in the T4 Dixons....)

I don't know where in the North you are, but I would leave Sheffield ish by 4.30am. Sounds early, but last month we went to Norway from Gatwick on an early flight and left at 2.30am. Glad I did as we had a blowout on the M1 in the ruddy roadworks with no hard shoulder so had to be towed off to change the wheel, adding about an hour and still made it.