? Yes you can. I use a Polish SIM card(with Play) and use it all year round in UK, except for the 2-3 weeks or so when I'm back in Poland for holidays.
While you probably can. The law is written to prevent use cases like this, to avoid a race to the bottom between operators in different countries. Which makes sense since infrastructure costs can vary largely between countries, so it'd be unfair competition.
Interesting that they do this because they know infrastructure costs vary between countries but they don't give a shit about the price of electronic goods, services, etc in the single market.
No, I just have a dual-sim phone(OnePlus 5T) - for data and calls in UK I have a proper British sim. But whenever I need to call my family or anyone in Poland, I just dial from the second(Polish) sim, and call for free, using the minutes that come with my Polish contract(and they can call me for free on my Polish number as well). It's also pretty useful whenever I'm somewhere in the UK that doesn't have a good signal on my main network - then I can use the Polish sim to connect to any network that has the best signal and just use data off that card temporarily.
Well, yes, it remains unseen what happens once brexit actually happens. But this solution works at the moment, and I can only imagine it can also work in other EU countries.
I can't get a polish SIM card and use it for an entire year in Germany, for example.