GPS based on satellites have been great for past wars. If we stop fighting those, or come up against an adversary not willing to fight past wars, systems based on fragile satellites will be a down fall.
Even having a fleet of CubeSats ready to go may not be good enough because of the easy with which the most common orbits can be filled with high velocity mines.
Maybe the UK should consider using the lunar orbit, but even there at least three nations have "easy" access so that orbit isn't particularly safe.
Enter eLORAN. An updated GEE system, UK only. Could be very useful except it uses stationary transmitters so would have to be harden against attacks. Might be a way to secure air and water after all anyone in the area with permission wouldn't mind if the GPS systems didn't work, they wouldn't be using them.
A better system would be one that could be installed quickly, relocated quickly, plentiful and cheap. Maybe bonfires on hill tops? Might just be what it takes to win the next big war.