Building Regulations
Currently there are no formal compliance standards for simply improving the thermal efficiency of your home own. A guideline does exist of achieving a minimum U-value of 0.16 W/m2K for a loft space. (This measures how well a building element conducts heat). So you have the freedom simply to top-up your loft insulation with any amount of Celotex.
This guideline stems from the Building Regulations governing the minimum standards required for energy performance. Known as Approved Documents L (2010 Edition), this is the primary vehicle upon which the UK government uses to drive through its ambitious carbon reduction targets towards zero in new homes and buildings by 2019.