Well, mixed Irish/Lancashire/Yorkshire family living in Lancashire. Got a lot of 'em.
As much use as ... cat shit in soup/ashtray on a motorbike/the cat I haven't got (The cat I haven't got also knows things that are obvious.)
A dog wi' a clawhammer up 'is arse could do it. (Said dog is a comparator for the utterly clueless, also.)
Your imagination's as big as your brain isn't.
Well, if $UNLIKELYEVENTOR_TRUTH, I'll show me arse in't co-op window.
Oh, he'll chew nails an' spit up rust! (For impressively clever/brave/obedient small children.)
Well, we could go down't butchers and see if he's turned the bacon slicer on? (Entertainment opportunities hereabouts seem somewhat limited)
You're like christmas comin'
You're shapin' like wet lettuce (not even reaching the standard of a dog wi' a clawhammer up his arse)
Don't talk wet! (I find your remarks foolish and inconsequential)
He couldn't lie straight in bed, that one. (He is lacking in honesty.)