Re: end UK dependance on GPS?
The reason you need an accurate clock is so that you know exactly where the satellites are in their orbit. The difference in the times reported by the satellites gives you their relative distance from each other to you (the signal takes longer to travel from the furthest satellite). As this only gives you relative distance compared to all other satellites being received, you need at least 3 satellites to compute a position (so long as you know your altitude). If you had an accurate local clock, you would only need 2 satellites.
The old "transit" satellite navigation system used the change in doppler shift of 1 satellite as it orbitted to find a position, but that depended on knowing the distance and direction you had moved while taking the readings (which took about 20 minutes to complete). A transit satellite would pass within range about every 2 hours on average (more frequently the closer you are to the poles). So you could only obtain a position about 12 times per day. So no good for precision real-time navigation.