Probable specific to Scotland and central belt at that
Penny a skelp - sounds horrific explained but actually real fun. If as a child we were cheeky we were thrown over and uncles shoulders and relatives were allowed to skelp (spank - but it was never painful or against our wishes) us for a penny in the cheeky jar! (This Money went to a local charity)
For a window you make a great door! - when you unwittingly wandered in from of the tv and blocked the view!
'Too many chiefs, not enough Indians'. My Dad always used that to describe the NHS. Another one he used 'not enough foot soldiers'.
Still true
Could not organise 'a piss up in a brewery'. If a person had all resources necessary but never got the job completed.
Tons of memes referencing this one in relation to the downing st Covid parties!
My gran used to say 'If it's for you it won't go bye you'.
Still one of my favourites
Sundays laundry's out! - your petticoat is showing! (Nobody but my mum wears petticoats now do they?)
My gran, if someone was being mean with money : "There's no pockets in a shroud."
One of my mums favourites this
My Gran always used to refer to "spending a penny" if anyone needed to go to the toilet. eek! I'm old enough to remember when this was true! Certain public toilets usually in train stations - a bloody quid now!!!
All fur coat and no knickers. This is how mum describes one of my aunties! 
It's not the cough that carries you off, it's the coffin they carry you off in painfully apt with Covid!
About as useful as a chocolate fireguard
Mine said of certain people/organisations (mainly the tories) "about as much use as a chocolate teapot at a garden party"
"If brains were gunpowder, you wouldn't have enough to blow your hat off."
My family's one is
If you had another brain cell it'd be lonely
@ThomasinaGallico that was used in our family even though my grandad had been a pow he never seemed to take offence though
@crackofdoom some things were superstitions as much as sayings, picking up a coin you'd dropped yourself - and depriving the find from sweeps and homeless - was considered greedy and unlucky among my lot
I sometimes say 'what you talking about, Willis?' which confuses my kids.
That's definitely an era specific one!
My mum also sometimes says (when a day hasn't gone as expected AT ALL) "I love it when a plan comes together" with a wry
anyone here get that one ?