My first thought was because many British bedrooms are rarely large enough to have large enough beds/multiple beds to fit many people as comfortably as people would like. There was a thread recently about beds for couples and many were saying that a double was too small to sleep both comfortably -- then add children into the mix with bedroom sizes as they are in many places.
My spouse and I transitioned our children from our bed to their own in their own room at about 11-15 months. The one we transitioned at 11 months was the one who would turn sideways and kick.
We did have a transition time where they would get unsettled at night, so we either set up a double bed (with our oldest) so one of us could sleep in there if needed or we had a mattress/soft chair that folds out onto a mattress on the floor next to their bed that we could crash on there if it was needed. Some took longer than others to resettle into this routine, but we found it lessened when there was a sibling in the room too so I do think there is something to the idea that young children prefer to sleep around others, just not always parents.
We might have kept them in our room longer, but there was really no space for another or large enough bed, especially when we got to multiple children.
Oddly, growing up, my sister and I regularly slept in the same room as our mother when our father was away for business/whatever, even did it when I was 13. But the rooms were much larger with more sleeping space and at times was the only room with air conditioning/a ceiling fan for the heat -- I've not been in British houses with similar space.
It certainly didn't make us more secure, and I can't remember actually wanting to do it - my mother would say that's what we were doing that night and that was that, at least by the time I was old enough to remember, it was far more about what she wanted.
When my oldest was 7 or so, he cried out from his room because he'd got his leg caught between the bed and the wall in his sleep. I cried afterwards because the same thing had happened to me at a similar age and I'd just laid there, in agony, all night, too scared to be a bother.
I think he was far more secure, independent, and willing to take risks than I was at the same age and I don't think it came from where he slept. It was a mix of many factors.