is a expelair cf20 fan suitable for a kitchen,and is it against the law for a contractor to install one?

Answer 1

I think the CF20 performance is more suited to a bathroom, a CF40 is an ideal fan for a kitchen, needs a hole through the wall for 100mm duct.

Against the law? No. You would need an electrical contractor to innstall it and wire it up to a new circuit.

Answer 2

I believe that kitchen fans must extract 40 cu/ft of air.
No its ok to put a fan in as long as you are replacing a fan or connecting to an existing point of connection.
Otherwise it's under Part "P" and all the legalities you have to follow.