A kitchen is a habitable room for the purposes of the Building Regulations. A habitable room is one of these:
A room occupied or designed for occupancy by one or more persons for study, living, sleeping, eating and kitchen if it is used as a living room, but not including bathroom, water closet compartments, laundries, serving and storage pantries, corridors, cellars, attics and spaces that are not used frequently or during extended periods.
Conservatories are not subject to BRs unless they are large, but you can't make it into a kitchen - not for safety reasons, but because it makes it habitable.
The definition of a conservatory is something which has a translucent roof (can't remember what proportion of roof.
Conservatories are often cheap because they aren't built to BR standards - although many are quite good quality structures, invariably they let a lot of heat out so don't satisfy thermal requirements of BR.
What you need is a kitchen extension, and that will have to satisfy BR in the way it is constructed.