If you'll be driving for long, get a copy of the "Highway Code" - a large newsagent, or maybe a bookshop, should have it. It's a standard handbook that apparently details all the driving rules (whether law or just common-sense or etiquette) about driving in the UK, although most drivers seem to forget it pretty quickly
There are some simple conventions on signs, so it doesn't take long to learn what each sign should mean, &c &c.
As everybody has said, try to book in advance if you want an automatic, though it's pretty common on the more expensive rentals. You might have to spend some extra time explaining your driving license to them if you don't have an IDL. Try to bring along some alternative ID
(passport?), and more than one form of plastic, just in case.
Compared to the rest of Europe, the UK is a little stronger on lane discipline. If you have more than one lane to choose from, keep to the left.
Speed limits? If there's no sign telling you otherwise, assume 30mph in towns, 60mph on the open road, and 70mph on the motorway. Don't worry too much about speed cameras - if you stay close to the law, you're fine.