The difference between US and British English – does it matter?

I recently went on holiday to the States and it got me thinking about the importance of the differences between US and British English. Let’s leave pronunciation and spelling aside, as they are fairly easy to cope with once you’ve had a bit of practice.
What I want to talk about this time is differences in words used and even cultural differences that have an impact on the language. Let’s start with one that you can’t help but notice on a fly drive holiday: give way signs. The signs look identical, a white triangle pointing downwards with a thick red border. But whereas in the UK these have “GIVE WAY” printed on them, in the US they have “YIELD” printed on them. Accordingly the language used to talk about obeying these signs is different. In the US (and for that matter, Canada, Ireland and South Africa) you yield to traffic, in the UK (and the other Commonwealth and English speaking countries not mentioned above) you give way to it. This isn’t a major problem though, as the other phrase exists in non-traffic uses in each dialect, everyone can understand the other’s phrase (especially with the help of the otherwise identical sign), even if it might take a moment or two to get it the first time you come across it.
Other words that I had to remind myself to use were “parking lot” or “parking garage” for car park, depending on whether I was referring to parking outdoors (parking lot) or in a multi-storey car park (parking garage). Another similar example is mailing letters in a mailbox or posting them in a post box. In the UK, we understand both sets of words (possibly because of all the US TV shows and films we get), but we tend to associate “mail” with email without further context. In the US, on the other hand, it’s easy to find people who don’t understand the UK terms, but you’ll also find people who quickly understand what you mean.
Nevertheless, there are some areas where most British English speakers won’t understand a US word or phrase unless we happen to have gained specific knowledge of that area. For instance, in 2001 it was reported that the 9/11 terrorists had used box cutters to hold up the plane. Although it was clear to people in the UK that this meant some kind of cutting implement, what wasn’t clear was that this was the implement we call a Stanley knife. In fact, I think most of us assumed that it was some special form of tool that existed in the US rather than the UK and most of us imagined something much larger and more sinister than a Stanley knife.
Other difficult areas include things that only exist in one country, but not the other. Here I’m thinking in particular of biscuits and gravy. A biscuit is basically a savoury scone (except the biscuit tends to have buttermilk and shortening in it, whereas scones typically contain milk and butter instead) and although the gravy is a meat-based sauce, it contains a great deal of milk and is therefore pale in colour and not like the dark brown sauce Brits usually mean when they talk about gravy. Another example is grits, which is made of ground corn, and is a bit like a gritty, savoury semolina. Although we’re capable of making such things in the UK (although in some case ingredients may be hard to obtain from British shops) these things are not well known – and we have the second problem that the word “biscuit” is already in use in the UK to refer to what Americans call cookies. Moreover, the “shortening” which I mentioned earlier is itself another example of this issue. In the UK “shortening” simply means any fat which is solid at room temperature and you can make pastry with (e.g. butter, lard, margarine). In the US, on the other hand, you can buy a product called shortening (or Crisco, the main brand for this product) which consists of hydrogenated vegetable oil and has a higher fat content than butter or margarine. UK supermarkets don’t sell Crisco or a product which corresponds to it.
Another example of things that only exist in one country, but not the other are designations of road (motorways, A roads and B roads in the UK, highways and interstates in the US) which don’t have an equivalent in the other country, not necessarily because similar roads don’t exist, but because they haven’t been categorised into the same groups. Likewise, various jobs and positions only exist in one country or have a different meaning. The US has district attorneys, sheriffs and realtors (not quite the same as an estate agent), the UK has the Crown Prosecution Service, chief constables and beefeaters. In some cases there are equivalents or rough equivalents, but in others there’s a completely different system and no equivalent exists. A lot of the time a speaker of the other English will be able to work out at least roughly what a person with that title would do, but a person from the other culture is unlikely to understand precisely what the role entails and in some cases the misunderstanding can be quite significant.
The other area that has the potential to cause major problems is when a single word refers to two different things in US and British English, but unlike with biscuit, most speakers aren’t aware that the meaning isn’t the same in the two languages. One such word is “peak”. In the UK the peak of a cap is the bit that sticks out the front and shades your eyes. However an American would call that the cap’s visor and would understand the peak of the cap as the highest point of the cap. Just to complicate matters even further, there is no difference between UK and US English when talking about the peak of a mountain, the peak of someone’s achievements or a voltage peak. With words like this it really is important to use the same English as your audience or you won’t communicate what you intended to.
One final difficult area is where there are words and phrases exist in one country, but not the other, even though the concept exists in both. This includes phrases such as “in the main” from UK English, meaning “for the most part”. Although all the words in this phrase exist in US English, the phrase itself is completely meaningless in US English. Other examples include the US words snarky (sarcastic, impertinent, irreverent) and conniptions (fits of anger/hysteria), which exist in US English, but not UK English. None of those terms are likely to result in a major misunderstanding, but they do make a sentence considerably less comprehensible for speakers of the other English.
Of course, most of the time the two forms of English genuinely are entirely comprehensible to speakers of the other, but every now and again problems will arise, at which point it pays to know your onions as we say in the UK or know your stuff as English speakers say everywhere.