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1. All mouth and no trousers – All talk, and no do
Example: "Ah, you shouldn't rely on that lad. He's all mouth and no trousers."
2. Argy-bargy – An argumentative altercation
Example: "Madame Pompous got into quite the argy-bargy last night!"
3. Damp Squib – Something that fails
Example: "Your brother is a damp squib when it comes to school."
4. Doofer – A British version of whatchamacallit
Example: "Will ya hand me that doofer?"
5. Earwig – To eavesdrop
Example: "Quit earwigging on my conversation!"
6. Idiot box — A television set.
Example: "Shall we see what's on the Idiot box tonight?"
7. Kerfuffle — A quarrel over opposing views
Example: "Those British politicians – always getting into silly kerfuffles."
8. Knackered — Incredibly tired
Example: "A day of field work always leaves me knackered."
9. Miffed — Offended
Example: "You really know who to get someone miffed you arrogant bastard!"
10. Odds and Sods — "Odds and ends."
Example: "I've still got some odds and sods to deal with."
11. Penny-dreadful — A cheap tabloid
Example: "I read about the apocalypse in the penny-dreadful."
13. The Fuzz — The Police
Example: "The fuzz are on to him."
14. Drinking Sherbets — Drinking Beer
Example: "Whose up for some sherbets at the pub tonight?"
Read more here:
https://www.smartling.com/blog/50-british-slang-words-phrases-you-need-to-know/
https://www.fluentu.com/blog/english/english-uk-slang/