The US do not allow the use of any other GNSS. They did not and do not like other nations having positioning systems that the US cannot switch off or control. That's their problem to deal with, although I understand that the EU has requested clearance for Galileo in the US. That of course gets tied up with trade issues.
Naturally it took manufacturers some time to update hardware and software to cater for Galileo reception, and existing devices cannot be retrofitted. Smartphone makers buy in GPS chips from manufacturers like Broadcom, and all GPS chips now made for smartphones and other devices fully support Galileo.
If the UK does decide to go ahead with its own GNSS, there will of course be a similar delay of a few years while manufacturers catch up. They may not bother to do so, since smartphone users will already be well served with existing systems including Galileo. The only applications for a UK system will be the military and a few specialised functions.