Thick as two short planks
Sandwich short of a picnic
Not the sharpest tool in the shed
Dumber than a box of rocks
Two bricks short of a full load
Tuppence short of a bob.
Not the full shilling.
Dim as a Toc H lamp.
Too daft to come in out of the rain.
Not Firing on all cylinders
Lights are on but nobody's home
---adding---
Not playing with a full deck
Not the brightest bulb on the porch**
**I think this one is American, I came across it while googling for the origin of the previous one, but I think I have heard it used a few times since I've been here
and here's some more musing on the possibility of a Brit/Yank difference that I came up with whilst composing a Facebook post to ask for help from American Friends which I then used to bully myself out of for the sake of not causing possible offence....
There could be an element that referring to people's possibly less than optimal cognitive ability could be considered discriminatory, which is something society is more careful about now than it was when i lived in the UK, so people in both continents are less likely to use these terms now, and I'm less likely to have heard them in America as this is where I've been more recently.