Hi Suitsya,
I live in the Midlands, just a few miles north of Birmingham. I’ve travelled to Heathrow for all bar one of my numerous JY holidays and have either driven myself or used National Express. My last experience of National Express (last August) was so horrendous that I never want to use them again, but I guess other people may have a different view.
As for flying down to Heathrow - I wish! There are no connecting flights from Birmingham to Heathrow so you’re faced with the choice of road or rail. And if, like me, you don’t fancy hauling your luggage on/off trains then you've no alternative but to travel by road. Of course, as Rose1999 says, taking a taxi is the most stress-free alternative but it is expensive. You may wish to compare that against the cost of fuel and car parking, and potentially the cost of an overnight stay and decide if it’s worth it. I guess you pays your money and takes your pick.
Cheers
Mike