Re: Receiving the signal
I believe for consumer purposes, you won't notice any difference if the UK is involved or not. Like you can already pick up GLONASS and everything else if your phone supports it, the core product for the public is out there and usable.
However, you need a phone that supports GLONASS, they aren't (I believe) transmitting a plain GPS (as in the US GPS system) signal, they transmit their own.
The next-gen of phones will likely all support GPS, GLONASS and Galileo. Whether it will make your satnavving any more accurate is questionable, really. It's supposed to but I doubt you'll see much practical difference.
The real problem is that we'll have to pay through the nose to use the interesting stuff (commercial, military, etc. for everything from planes to cruise ship).