I recognise the telephone number - its the AirOps Room in Faslane, Scotland, been well published in NOTAMS before.
GPS is jamable! Put enough nonsense into the receiver and close enough and its gonna stop working.
It uses freqs in the 1.5 and 1.2 GHz bands - yes NORMALLY line of sight use.
Don't forget radio range vs altitude means a larger coverage area. 320NM - dunno what that equates to ?
where does the lat/long put it?
Some where in the Clyde estuary would be my guess!
regards