That's why there are no GPS let downs authorised (well, not without other systems like baro altimeter or with quite a high minima) and why FMC automatically disregards GNSS position information in favor of IRS + ground external systems.
a stunning piece of information - maybe it's true in the UK, but there are hundreds (probably thousands) of GPS "letdowns" that have been in use over the past 15 years or so in countries that are not the UK- and there are IRSs that will align in flight using GPS reference...
