At the time of writing most UK 5G networks only offer 5G roaming within the EU, if at all. Though there are exceptions to that, such as iD Mobile.
Beyond that, EU roaming is free on some UK networks (though most have their own fair use limits), whereas outside the EU it’s variable. But in most non-EU countries on most networks, you won’t get free roaming. It’s always worth checking the roaming terms for your destination before you travel.