Drive defensively

Drive carefully and defensively

I cannot stress this enough: Assume the other drivers have a better idea of what they're doing than you do, and take your hints from them.

One thing to note: Most European drivers are much more aggressive than American ones—not as much in the U.K. as on the Continent, but still.

Be prepared—and, please, leave the road rage at home.

Driving links

Gov.uk - Official UK governmental rules of the road, guides to road signs and their meaning, and more
Amazon.com - A link to get road maps of Great Britain.Partner
Slowtrav.com - Nice article introducing all of the quirks of driving in the United Kingdom—not just how to drive on the left, but roundabouts, highway types, speed cameras, and handy tips.
Orcutt.net - Almost comically in-depth blog entry—with loads of links—about driving in the U.K. This guy went way above and beyond what's necessary when planning to drive on his British vacation, but it is about the most complete explanation of being on the British road from a Yankee's point of view as you'll ever find. Some good tips.
Theaa.com - The British version of AAA provides, among other things, a monthly gas prices chart and a map of traffic incidents and slowdowns.
Parkopedia.co.uk - live parking maps
Tfl.gov.uk - Driving in London
Cclondon.com - All about the infamous "congestion charge" you have to pay if you happen to drive in central London.
Urbanaccessregulations.eu - Central clearinghouse for information on every city in Europe (most, nowawdays) that has some sort of limited access to the city center, congestion charging, Low Emission Zones, or other wrinkle you need to be aware of.