A nice compilation of British euphemisms by The Economist, including:

What the British say: “With the greatest respect”
What the British mean: “I think you are wrong (or a fool)”
What is understood: “He is listening to me”

Now of course just about any language and culture has its euphemisms and they are of course particular towards the specific group they are used in. What makes English or British euphemisms in a way special is the widespread use of English in many international environments such as business or science. This widespread use invites misunderstandings as with the example above. Having worked in a multinational scientific environment for more than 15 years the one thing this has taught me is that euphemisms, irony and sarcasm are best avoided in situations where non-native English speakers, including Americans (note the sarcasm), are involved. It’s just too frustrating and your counterpart will just not get it, in the best case that is. In the worst case they will get angry or think you are just plain arrogant.

I have learnt that euphemisms and irony are best reserved for situations where you are either amongst good friends or with people of the same background, especially the same language background. Or in situations where you want to let off steam and cannot i.e. with your non-English boss.