FF
Can you be sure that the outage was related to your physical position over the ground? I have witnessed a similar simultaneous outage of completely independent GPS but am not convinced that it was caused by external factors.
Might there be a relationship with on-board equipment? A change of radio frequency, Nav aid, squawk? Or (surely not) mobile phone left on? Portable GPS are bound to have inferior signal/noise performance than installed equipment, and of course the antenna might actually be inside the cockpit.
Such a geographical outage would surely lead to much howling from everyone from boaters to haulage contractors. I believe there is published information on GPS outages (It's referred to in the recent CAA 'stick to reliable ADF' report) so perhaps someone can supply more information on the status on your date/time.
David