Here in England, the safest leg-covering clothing option for an American is jeans. It helps that it is now rare to be turned away from a those must-see tourist traps more broadly known as abbeys and cathedrals for wearing clothing deemed inappropriate. Though it is still possible that you'll be turned away for a lack of care when dressing to enter a place of worship, if only to get that last minute rubbing of your sixteenth-century ancestor's grave. Still jeans are safest for you American's taking a holiday (vacation) in to this particular mother country.
They'll save you from announcing to your tour guide that you've got to run back to the hotel to change into a clean pair of underwear or your taxi driver that you've soiled your knickers on an early morning walk. No matter how well you command the English language, both are really just not done above a whisper in any country. You may find it easier to pack clothing you like to wear and delete the term 'pants' from your vocabulary entirely.
The English only use the term 'pants' to refer to underwear. They say 'trousers' where Americans will use the term 'pants.' Since American films and tv shows are so prevalent here, the English can tell you all about that particular difference in language. When faced with a real American demanding a cloth to get the coffee out of their pants, they'll still hear underwear. They'll still look at you quizzically, and they'll still bring you that cloth. The English are, when faced with rudeness (why are you mentioning your pants in public), always polite.
They also cut women's pants a wee bit differently than in the US. More on that problem, with pictures, on Monday.
Edit 22 January 2011: I have been informed that in the North folks do refer to trousers as pants. I must now consider moving.