I have never flown long haul and only ever flown on Monarch type planes with no legroom. I am clueless about planes so don't even know the type of plane I've been on.
Anyhow, to get to the question. Is it much better on a Virgin flight, economy, Heathrow to Johannesburg? The thought of not being able to sit down with my feet on the floor because there is not enough legroom for 14 + hours fills me with despair. (I'm 6'4).
How much more should I expect to pay to fly business class?
You`ll be more comfortable on a scheduled airline like Virgin than you will be on charter. You could do the happy medium and fly Premium Economy on Virgin or World Traveller Plus on BA. They are intermediary classes somewhere between Economy and Business and a happy medium. The flight to JoBurg is about 11 1/4 hours.
In Virgin Economy the usual 31 inch pitch for your seat, upgrade to premier plus and that goes up to 38 inch pitch, obviously dearer so depends on how much you are willing to pay at the end of the day, I am 6' 2 and have done a nine hour plus flight in a 31 inch pitch seat and it was ok. Just remember to get up and have a walk around every couple of hours, Enjoy. You will be fine,plenty to do and see on the flight which will take your mind off it.
If you can afford it, go Business Class, more leg room, more pitch on the chairs and width, better food and wines, more comfort. There can be some deals floating around and it's worth exploring first class for such deals too, sometimes cheaper than flying business.
the difference in jet lag is quite remarkable, the one advantage that you get with Joburg being that there isn't that much change, just the length of the flight.