help me plan a road trip!

Those timings look fairly realistic (I'm intrigued by the idea of visiting Orpington. I used to play squash there when I lived with my parents, but it doesn't have much else going for it). If you decide on the Brecons instead of Cardiff, you can come back east on the A44 which is a nice road and skirts the Cotswolds, so you could always divert for a couple of hours to tick that one off.

Your drive times are far too optimistic unless you are driving in the middle of the night!

Monday - you could visit Brighton or Lewes en route to Eastbourne. There are some pretty villages such as Alfriston.

Tuesday - today, I have driven from near Barnsley to Tunbridge Wells in Kent on the A1. It has taken me nearly 6 hours of driving and Eastbourne is another 45/60 minutes from here. I do this drive regularly and it can be done in 4.5/5 hours if no delays, but there are often delays. You aren't going to have much time to see much of Kent. What do you want to visit in Kent?

Thursday - you are going to be spending all day in the car and will turn the trip into a marathon slog. You don't say how old your son is, but if not an adult, he may get bored.

Friday - are you interested in old stone circles? If not, drop Stonehenge. It wouldn't be in my top 100 things to see in the UK, but it's your trip, not mine.

These last two days could be used instead to make a more sensibility paced overall trip. It's difficult to advise as you don't state your interests. From South Yorkshire, you could have a day in York. Spend some time in Kent. Visit Rutland Water just off the A1 and take a boat ride. Visit the Georgian town of Stamford en route or Burghley House. Visit Lincoln. Many interesting options that all involve less driving.