British English is quite different from, for example, American or Australian English. This is a fact that the British love sharing with newcomers -- typically within ten minutes of having met them for the first time.
They will do this, either by implying that "all other English speaking countries speak a dumbed down version of their beautiful language", or by more politely claiming that "the British and the rest of the English speaking world are as cousins divided by the same language". The main reason for this is that that British often prefer a slightly more formal and eloquent spoken language than their foreign counterparts. However, with all of the different British dialects, this type of statement is often said with a twinkle in the eye.
Another important difference between 'international' English and British English is the British tendency for self-deprecation. Compared to many other western cultures, the British tend to understate many things they do or say. This also means that any type of boasting often will be met with the legendary British sarcasm or irony.
Not Indecisive, Just...
Because of the polite social rules of the country, many British have a tendency to avoid being too decisive. This can be seen in their very frequent use of the term "ish". The term can be translated in to something akin to "approximately", "roughly", "more or less", "so so" etc., and is usually used to soften a commitment or for example criticism. Examples of this are; when the British agree to meet someone seven-ish (rather than commit to meeting at seven sharp), or that they think something or another was great-ish (rather that great or half bad).
The British eloquence and at the same time indirect and polite use of the language can at times be frustrating for cultures that are more forthright and direct (for example some Middle Eastern, Germanic and Nordic cultures). However, as the correct use of the language is very important to the British you should always be careful what you say and how it is said.
Having said this, most British speak English as their only language. Because of this many of the people you meet are impressed by people who speak English as their second or third language. This means that when speaking with foreigners they will not pay too much attention to the quality of your English as long as they can understand you. In fact, as a foreigner you will often be told that the mere fact that you have gone out of the way to learn their language is greatly appreciated.