Rusty trowel is right in one way – but not in another.
Smart casual is Clarkson style. Its an abomination.
However no one should ever go out in Clarkson "smart casual" Its only for the sad dad or drunk uncle
If you have jeans – it should be trainers or boots and a t shirt or polo shirt if you must. If you wear a dress shirt you should have a tie. If you want to wear real shoes then you must have pressed trousers.
I would either wear t shirt, jeans or chinos and trainers / deck shoes jumper or sports jacket t shirt dress trousers polished shoes.
Jeans / untucked dress shirt / polished shoes / jacket should be grounds for getting the AK47 out