Floor U-value limits for dwellings are set in Technical Guidance Document L. Ground floor heat loss depends not only on the insulation thickness but also on the ratio of the floor's perimeter to its area, so the calculated U-value varies with the shape and size of the building.
Ground floors lose heat mainly around the perimeter, where the floor edge is closest to the outside. For that reason the calculation of a ground floor U-value uses the perimeter-to-area ratio: a small or elongated building has proportionally more edge and therefore needs more insulation to reach the same U-value than a large compact one.
Edge insulation and the continuity of insulation between floor and wall are critical. A well-insulated slab with a cold edge detail can perform far worse in practice than the calculated figure suggests, and it also raises condensation and mould risk at the junction.