Another couple of points to consider:
The parking places are probably smaller than you're used to. There was even an news segment on the BBC this morning about how cars are getting bigger and yet the spaces were laid out years ago for smaller cars. Parking itself can be an additional expense.
Gas is about $8 a gallon. Get the smallest, most fuel efficient car you can.
When you're in an emergency situation and/or you're reacting instinctively, your first inclination will be to jerk to the left as in the US this would put you on the shoulder and out of harms way. In the UK, you'll be driving head first into oncoming traffic. I did exactly this in a store car park...thankfully it was rather empty; I was unwittingly playing chicken with a guy coming at me who had no idea why I suddenly swerved across in front of him and stopped dead perpendicular to the space lines so he passed by on my left. It was rather scary for both of us I'm sure.
I still haven't figured out large roundabouts (smaller ones, fair enough, but more than 2/3 lanes and my head explodes.) I accidently cut off a policeman when i went around one more time and was helpfully informed by our boy in blue that I had been in an exit lane (what? where?) and by going around one more time I had caused him to swerve. I profusely apologised, took the rental car back, and everything i've done since has been on public transport.
There are speed cameras all over. Do not speed. Seriously.
As a driver, I found UK drivers to be much more considerate than in the US. For the most part, they'll let you merge, let you turn, and exit, and will be generally much more friendly.
And on the other side, as a pedestrian, I find drivers in the UK to be incredibly hostile. The thought seems to be that you're in the wrong place if you're in the road at all (with the exception of zebra crossings - they HAVE to stop.) Unlike in the US where drivers will almost always give way to pedestrians (especially coming out of grocery stores, etc.) the ones here will drive at you. It can be rather intimidating - just be aware and look both ways.