To answer the original question, yes, it would be a major issue, and the usa have the capability of turning off the public system, reserving it for their military. The Eu with Britain's help started building the rival Galileo system, but since Brexit, our involvement and access is no longer guaranteed, and the UK is currently looking to go it alone.
From the perspective of us as small boat owners, we should all have the tools and skills to find a safe harbour without electronic aids. Even in fog, the combination of compass and paper chart, from a known or estimated position should be good enough to find Bournemouth Beach, for example, then follow the coastline into Poole.