The one piece of advice my brother thanked me for offering after he visited the UK was to not bother hiring a car. The one thing I did forget to mention was that when looking both ways to cross the road, look RIGHT first.
When they see a sign in someone's drive advertising hardcore, tell them they'd do better watching Channel 5. Oh, but DO warn them about the TV.
Ask them to try to not laugh when passing the butcher's shop window when they're advertising faggots for sale. They should at least once try spotted dick, if only to tell the folks at home they did.
Hmmm, road signs. A sleeping policeman is not really a sleeping policeman. They need to stop at Zebras and Pelicans (or is it just Zebras?) when someone's there. Speed limits are NOT marked if they coincide with what the general rules are, so they need to know the difference between a dual carriage, a lit in-village road, etc. Oh gads, I could go on and on about the driving differences. You sure you want them to drive over here?
I think the one big thing that bothers me about American tourists...to the point that I try to disappear into the woodwork when they are around...is just how loudly they speak, which makes any well-meaning question sound derogatory.
The biggest thing that got me in trying to adjust here was how backward everything was. (Oooh, that doesn't sound right! I mean...opposite.) If they were to keep in mind that one simple rule "expect it to be opposite" then perhaps they might not be too surprised by the little things. I always felt as though little tricks were being played on me, like in Amelie. Things like salt and pepper shakers, toilet handles, light switches...I mean, it's really the bizaarest thing! Like in words...pasta/pasta, yet tomato/tomato, or the way things are
spelt spelled. (I once had a work-checker spit out "airport" and I'll be darned that I could even find it in the British dictionary! *doink* aeroport!) So...expect the little things to be completely opposite. Knowing that, they'll be just fine.