Isn't it the most accurate and reliable bit of equipment that even a simple UK PPL can use for navigation... Then why does the mighty US Air Force put restrictions on it's use above the restriction thay place on a simple VOR or ILS?
Because of the issue of
availability, not
integrity. Sometimes GPS isn't available in a way that meets its integrity standards, and it tells you it isn't -- in part because those integrity standards are so high. Thus there needs to be a different sort of approach available.
Availability vs integrity is a trade off. The higher the standard you insist on (and for GPS for even NPAs, the ICAO standard is many orders of magnitude higher than for conventional navaids) the lower the availability. With an NDB, if it's radiating, it's available. That's because the integrity of the navigational output derived from ADF/NDB is so poor.
If your point is that a pilot should never rely on the availability of a single navaid in a mission critical situation, I'm with you 100%. If you're suggesting that the USAF policy brings other aspects of the "reliability"of GPS into question, or its accuracy, then you need to rethink your understanding of failure modes.