Gobsmacked
"I was gobsmacked when I found out England won the world cup!"
Shocked and surprised beyond belief
Chinwag
"Those two are having a proper chinwag."
A good chat, catch up, or gossip with someone
Not the sharpest tool in the box /
Not the sharpest knife in the draw.
Stupid or silly
Living under a rock
He must have been living under a rock.
To unfamiliar with something that most people know about
Slag off
He was slagging off the staff.
To talk badly about someone or something.
Chuffed
"I'm feeling proper chuffed after that test!"
Overjoyed; full of pride
Damp Squib
He tried to win the race, but in the end, he was a damp squib and came last.
Something which fails on all accounts
Off your rocker
"Don't talk to that guy. He's off his bloody rocker!"
Mad or crazy
Full of beans
She was laughing and shouting and full of beans.
Energetic, lively, or enthusiastic
Bird
"Mate, do you see that bird over there!"
Girl or woman
(Impolite)
Gallivant
The boys went gallivanting around town on the weekend.
To go out to have fun
Butchers
"Have a butchers at that Ferrari!"
Look (Rhyming slang)
Butcher's hook: look
Poxy
"What a poxy restaurant!"
Lame or bad quality
Blimey
"Blimey! Would you look at the time!"
An expression of surprise
Quid
"I've only got a few quid, can you buy me dinner?"
British pounds (£)
Fortnight
I would love to spend a fortnight playing Fortnite.
Two weeks
Spend a penny
You can't get to sleep when you need to spend a penny.
A polite euphemism for going to the toilet to pee
A face only a mother could love
It was unfortunate, but he had a face only a mother could love.
Very ugly
To have a bee in your bonnet
"Wow! What's wrong with him? Seems like he's got a bee in his bonnet!"
Annoyed
To leg it
"I legged it from the police."
To run away fast
Bonnet
"Look under the bonnet and you might be able to see what the problem is."
The front of a car
Hood (US English)
Safe
"Safe for that, bro."
Thanks
(Also, hello, goodbye and trustworthy)
Wazzock
"Why did he say that!? What a wazzock!"
Idiot or fool
Not all it's cracked up to be
That film wasn't all it's cracked up to be.
Not as good as people are saying it is
It does not meet your expectations
Trollied
"Mate, you were absolutely trollied last night!"
To be very drunk