@Schnoofard There is a definite move towards phones being banned in day schools in the UK, across both state and independent sectors, and the majority of boarding schools effectively do this too - no phones allowed during the school day, which ends at 5pm. However, one of the main reasons why Eton can enforce a total ban is that they have literally no day students, whereas the vast majority of UK boarding schools have either a significant minority of day students or, in many cases, boarders are the minority. That makes it much harder to enforce the ban "after hours" because the day student parents are unlikely to also enforce it, so it creates exactly the situation you want to avoid, whereby the boarders dont have phones but their day friends all do.
The way I see it, a UK boarder is likely to be on their phone a lot less than a UK day student, but I really think the best you can hope for is ban during school day and limited during evenings and weekends. When they are at home, which many are most weekends, they are likely to have smartphones.
They do all also have laptops, and would likely be on those more in the absence of phones - school wifi content blockers block dodgy stuff and usually social media, but DS is as likely to be checking the football news as that anyway- the house fantasy football prize is much coveted :-)