driving in a place where everything is backwards,..... Really, it's much harder than you might think...... If you slip up once you could be killed. No joke.
Sorry, but this IS a joke. The UK is one of the safest places in the world to drive. As long as you don't drive recklessly you'll be fine.
Although we like to moan about the standards of driving here, the reality is that the UK's drivers are on the whole experienced, capable and a key reason why our accident statistics are so good.
If you're going to hire a car here, make the effort to learn about our driving practices so you know what to expect and plan your journeys in advance. Sat-Navs and Google street view should sort out navigation issues and ensure you aren't surprised when driving. Google's driving time estimations can be very accurate here, so that could aid planning too.
Driving in London is not much harder than most other major cities, but poorly sign posted and parking is limited and expensive, so stick to our public transport unless you like a challenge. I'd also suggest trying to avoid using the underground with children in the rush hour as it's rather intimidating for them.
The OP doesn't say how old or what gender his children are and that doesn't help us suggest the best trips. For example the recommendation to visit Cambridge is OK, but adding Duxford air museum would be great for a boy, maybe less so for a girl. On the other hand a girl might enjoy Blenheim Palace on the way to Oxford more than a boy. Oxford also has a good couple of museums when it's raining (in April it will rain sometimes).
If time's limited I wouldn't bother with Stonehenge. It's a long way out from London, you can't really get close to it and it's packed with tourists.
A trip on the Bluebell steam railway could incorporated into a trip to Brighton, not the Harry Potter line, but still an English steam railway. The i360 in Brighton ticks the 'wow' factor too.
