I own a building with 4 studio flats on the ground floor and four flats on the first floor. The building has a communal hallway on the GF with an external door at each end and a communal landing on the first floor.  Do I need to have FD30 doors on each studio or will conventional UPVC front door styles be ok ?

It's a legal requirement. The flat front door's have required to be fire doors in law since the 1960's. If you are putting new doorsets in they should be FD30S and in all cases must have self closers fitted.