1. Different social attitudes: The british are, in some ways, less conservative than Americans - the drinking age is 18, not 21, homosexual civil unions are legal, and abortion hasn't been a serious political issue in years. Mixing religion and politics is extremely rare in the UK. On the other hand, there are stricter controls on some things - it's much harder to get a driving licence, and the age requirement is higher than in most states, there are speed cameras everwhere, CCTV cameras in many public places (although they may or may not be working), and carrying weapons or other objects for self defence is illegal (in particular, note that pepper spray is illegal).

Getting into a debate about the right to bear arms will be very time consuming, and may also lead people to think that you're a gun-nut before they've got to know you properly.

Smoking in enclosed public places is illegal - that includes pubs, the underground, train stations... more or less any non-residential building actually... (similar to New York, so not so much of a cultural difference there).

2. The British apparently have a wider vocabulary of swear words than Americans. While most Britons will use them fairly freely in a social setting, things are a lot more reserved in formal settings talking to strangers.

3. Pubs. I've not yet encountered an American pub which bears more than a passing resemblance to a typical British pub. People might go to a pub just to drink and socialise, have a pub lunch, or as part of an attempt to get alchohol poisioning (otherwise know as a pub crawl), as well as to watch sporting fixtures (which seems to be the main purpose in America). Going to the pub does not usually imply getting drunk. Although the drinking age is 18, bartenders are supposed to ID anyone who looks under 21.

4. Roads. Though I doubt your friend will do much driving, he should be aware of the following things: Most cars in the UK have manual transmission. Cars are smaller and more efficient, petrol is more expensive. Speed limits are higher, but enforced more rigourously. Driving in London is not worth it. Central London has a congestion charge, and is very congested, and the major motorways around London are thick with traffic at peak times. Roundabouts (traffic circles) are very common and are used at motorway junctions. In rural areas, it's not uncommon to drive down narrow single track roads with tall hedges on either side.

5. Educational stuff: The UK education system works differently, as he's probably aware. Asking people about their High School GPA Major or Minor will make people think he assumes that things work the same in the UK as in the US. In the UK, High School is roughly equivalent to secondary school and college (college is not used to mean university). Important exams are GCSEs and A levels, which are graded with letters and related to specific subjects. There are no majors and minors in the UK university system - there's just whatever degree you're studying.

这是比较详细的版本，可以自己删改一下，或者google一下difference between british and american culture，会有更多的资料。希望能帮到你！