As others have said, driving in the UK is slow at best. Unless you are near an A or M road, count on no more than 35 mph - not only are the smaller roads twisty, narrow, and occasionally full of sheep, but they frequently go through small villages and towns with congestion. I remember one trip through the Yorkshire Dales and getting stuck in a traffic jam in some small village - it would have taken about 1 minute to drive through it normally, but was a good 20 minutes waiting on traffic.

If you are IN London, forget the car and use the tube or busses. If you are exploring the countryside, pick the car up at an outside location so you don't have to deal with London traffic or the congestion toll (for London city centre traffic). If you are traveling to, say,Edinburgh, a train would be a better idea.

The cars are better if they are small, as the roads are often narrow. Often the hedgerows (which cover stone walls) are right up to the edge of the road, and a big truck is coming the other way going much faster. It can be harrowing!

Automatic transmission cars are twice as expensive, normally, as manual. However, as mentioned, you'll get better gas mileage than you are used to. Also, don't pass up a chance to fill up when out in the country - the next petrol station may be far away, or closed (especially on a Sunday).

Driving in the UK is part of the adventure, IMHO