Maximum roof U-values for dwellings are set out in Technical Guidance Document L, with separate values for new build and for upgrades to existing roofs. Roofs are usually the easiest element to insulate well, so designs often exceed the minimum standard because the additional insulation is inexpensive.
The required maximum U-value is published in TGD L and differs between new dwellings and existing roofs being upgraded. Because attic insulation is comparatively cheap and easy to install to a good depth, many designs comfortably exceed the requirement.
The detailing matters as much as the thickness. Insulation must be continuous at the eaves without blocking ventilation paths, and where a room is formed in the roof, the build-up must control condensation risk as well as heat loss.