You think you know English? You don't know English.
Compared to the British, Americans are newbie English users. In British hands, the English language is a tool of infinite subtlety. For example, when a Brit tells you that they “agree with you to a point”, this doesn't mean that, with the exception of a few aspects, agreement has been reached; it means they don't agree with you at all. The magazine The Economist once reported that English-speaking Dutch diplomats had to employ British translators in order to understand what their British counterparts were really saying. This adds a whole new dimension to arguments at the pub.
A Chicagoan living in the UK finds that, although English is spoken here, the UK is still a foreign country. This is his field guide to help fellow Americans navigate the often delightful and sometimes puzzling cultural differences they'll stumble into over here.