Britishisms are expressions peculiar to British English, including is British idiom. See also List of British English words not used in American English. The word Britishism is an Americanism.

• "[It's] all gone Pete Tong" - all gone wrong (new-ish rhyming slang)
• As much use as a chocolate fireguard (or teapot) – useless.
• BBC English – the version of Received Pronunciation once considered typical of the British Broadcasting Corporation. Today regional dialects are frequently heard on the BBC.
• Devils on horseback – sausages covered in bacon
• Egg-cosy or -cozy – meal-time egg warmer, usually knitted.
• [it's a] game of two halves - literally a football match in which the two halves had very different characters; metaphorically, roughly equivalent to "It ain't over 'til it's over"
• Go pear-shaped, as in "It all went pear-shaped." – go wrong
• Hit for six – to hit mightily, to trounce (to hit a cricket ball off the field without a bounce, scoring 6)
• Industrial action; strike or work-to-rule by employees
• Lovely jubbly; great outcome, popularised by a catchphrase in a BBC TV programme Only Fools and Horses.
• Made redundant - laid off, downsized
• Not cricket – not fair
• Not much cop. – Of no consequence.
• Nowt so queer as folk – people are unpredictable (uses the Northern English "nowt" meaning "nothing").
• Pukka; good, an expression from the days of the British Empire in India (pakka = ripe in Hindi).
• Swings and roundabouts – gains in one area will equal losses in another (short for "what you lose on the swings you gain on the roundabouts")

Caveat lector: these are often very slangy, quickly obsolescent, age-specific and regional terms. Saying "Nowt so queer as folk" in a Cockney accent will get you mentioned in every conversation in that pub for the next six months. And quite possibly local radio.