Re: Receiving the signal
"For now, all the separate systems require a separate receiver, so you need to check which ones they can actually use. GLONASS support is fairly common in addition to GPS, and Galileo isn't too unusual, so it's quite possible a current phone will work them all. Beidou and NAVIC are currently only regional, so you're unlikely to find support for them in phones not sold in the relevant regions."
Actually, if you look at the specifications for the available smartphones, most of the new ones have support for A-GPS, GLONASS and often BDS (see phonearena or gsmarena for specs on many phones). A few phones can receive Galileo, but it seems to be fairly uncommon.
I have some hopes that Galileo may be added to that list, but currently chipsets and integrated tri-band antennas for the three bands used by the A-GPS, GLONASS and BDS GNSS systems are widely available.
Given that there are a billion or more people in China buying smartphones, and most of the phones are built there, I expect that those three will remain the 'standard' set. It is possible that the Chinese government may even quietly encourage all phone manufacturers to include BDS.
BeiDou-3 currently has 9 satellites up, with 35 planned by 2020, giving global coverage. India's NAVIC and the Japanese QZSS are regional and likely will be ignored by most chipsets and most phones sold outside those regions.
If Galileo has sufficient advantages or can be added at insignificant cost it will likely become the fourth 'customary' GNSS in most devices.
A hypothetical British GNSS would probably come in after the other minor systems, given relative populations and sizes of economy... in other words, unless it can be handled by a receiver designed for one of the three or maybe four major systems, it will be a regional specialty item in phones and GNSS receivers.