Considering that if the Yanks turn off GPS (or the Russians theirs) it will be in the event that you or those near you have been designated an enemy worthy of serious military action, I think you'll have a lot more serious things to worry about other than your in-car navigation! Please loosen up your tinfoil hats and let some blood to your oxygen-starved brains, I can't recall a single incident of the US actually ever having turned off GPS for UK users. In fact, due to the special relationship, it is highly likely that if the UK ended up in a shooting war with another US allie (say Argentinia again), the US would still favour us over the opposition and still give us GPS use. The Galileo proponents have talked up the US control of NavStar from the start with absolutely no basis in reality, and the tinfoil-adorned sheep have happily bleated on about it ever since.
On the other hand, I expect much more threats and controls on the GLONASS system, much in the same way as Putin has used the threat of withdrawing oil and gas supplies to his neighbours. Anyone that signed up exclusively to the GLONASS option would need their head checking unless they are already in the US naughty list (like Iran, North Korea, and other cuddly states).