Maximum window U-values for dwellings are given in Technical Guidance Document L, and the figure that matters is the whole-window U-value including the frame, not just the glass. Suppliers should provide certified whole-window performance data for the specific unit size being installed.
Manufacturers sometimes quote a centre-pane U-value, which describes the glass alone and is always better than the performance of the finished window. Building regulations and DEAP calculations use the whole-window U-value, which includes the frame, spacer bar and the edge-of-glass effect, so this is the figure to request and record.
Installation matters too. Even a high-performance window loses much of its benefit if the reveal detail creates a thermal bridge or if the perimeter is not airtight, so the fixing and sealing detail should be designed rather than left to site.