First do not use Polystyrene. Not sure in a domestic situation but insurers get very twitchy in industrial environments. Some very nasty fires.
Celotex and Kingspan both make decent PIR panels, if you want a little more insulation get the foil faced variants, there is a place in mid wales that takes the Kingspan seconds.
Alternatively you could try Rockwool or Knauf rockfibre slabs, but probably a bit pricier, and certainly heavier.
Choice either lay the boards over the ceiling joists or between the roof joists or both.
The PIR foams are far more efficient than glass/rock fibres, thus a roof build of 220mm glass is c 100mm of board.