There's multiple levels of GPS for different purposes.
GPS, GLONASS and Galileo all have "public" access levels that are reasonably accurate.
Which is fine if you're getting directions to drive a car between large areas on well defined routes, accurate enough if you're recording a walk, cycle or run and good enough to send a helicopter to find you if it all goes pear shaped
BUT... the public access accuracy of those systems is no where near good enough when you're pointing a £1m worth of arsenal at a battleship in the middle of the Atlantic; to avoid a very expensive splash you need considerably better accuracy.
That's what the UK is losing from Galileo due to Brexit.
Obviously GLONASS is not an acceptable alternative unless Cummings really is a Russian agent, in which case we're Donald Ducked.
Which leaves the British military forces entirely dependent on being friendly enough with the US to get better aiming accuracy than others.
The US don't give us or any other NATO member full GPS access either they keep the highest accuracy levels to themselves.
It's all part of the traditional story of British resting on their laurels (of having a smashingly good land based radio navigation system) while the Americans, Chinese and Russians produced better and trying to claim the flag on the sticker meant it was brilliant...
From a consumer level the only real issue with consumer devices using SatNav systems is when the owner deliberately offsets it by an amount known only to them (which usually means they're about to blow something up and don't want the enemy to use the same system to hit them); realistically this is only a consumer problem if you're on the Western Isles or North West Scotland during Operation Join Warrior.
If you're running around a desert with assault rifle on the other hand...
