>I don't think GPS is used by submarines to get accurate launch location information
Don't know why you think that - it's among the primary purposes for which it was created.
>Test flights of the Trident missiles used GPS payloads to calibrate and test the inertial guidance system.
Nope, GPS updates are received throughout flight - in fact Boeing have made much of the improved GPS unit they'll be adding under the contract they won last month to update and maintain the navigation subsystem. Reliance on GPS is of course a non-issue in the US......just a little problematic for UK claims of operational independence.