Some EU laws apply in Northern Ireland - I don’t know about laws in this area specifically. If a phone is legal in the EU, the UK may be required under the Northern Ireland Protocol (and the new “Windsor Framework”) to allow it in Northern Ireland. If they have to make a phone legal in Northern Ireland, it will be very hard to ban it in England, Scotland and Wales
Sure, but its not that hard or expensive to hop a ferry/train/plane trip to France or Ireland and buy an unlocked phone, as one example. Any laws the UK passes have to account for the geographical realities of its location.
Just because the UK is no longer in the EU doesn't mean its not affected by many of its decisions, given the enormous volume of trade that will continue to occur between them.
I hate to break the bad news, but people break laws all the time, including in the UK. People will just manage the risk exactly like any other law they choose to ignore.
Criminals still carry blades as weapons in the UK, despite it being a strict liability crime and at risk of stop and search.
And the Channel between England and France is a mere 20 miles at its narrowest point, Ireland is 12 (the UK's nearest EU neighbors). You can argue it down to zero miles for Ireland if you start your journey in Northern Ireland.
The sun absolutely does set on what little is left of the "British Empire" in the 21st century.
The famous phrase was as much about the relative geo-political strength of the empire in the 18th and 19th centuries as much as it has to do with actually being able to see a sunset... but feel free to take it literally if you like.