Joe, as you are only carrying out improvements to an existing installation ,which i take it does not include the building of an extension...loft conversion etc,then the building regulations do not apply in relation to the design of your intended fire alarm system.
The recommendation ,however, is a minimum of installed interlinked smoke detectors to ground floor hall and first floor landing.
If building regulations did have to be complied with then you would probably also have to install heat detectors in kitchen and main lounge ..again all interlinked.
spark1