I suppose my colleague was a summerborn, because I remember how surprised I was when he told me his age, and I combined it with few other anecdotal pieces to imprecise observation.
I did try to check that before posting, but both Reddit and Wikipedia gave me maps where the whole UK was marked as 17. (Looking at them now, they aren't really trustworthy, for my country they show 16 which would be true for someone in special needs vocational track, not in A-level equivalent track)
Anyway. It still means that in standard situation British teens leave school one year earlier than I am used to (school year when people reach 18/19), and with an epidemic of deferred school entry we face now (we have the same cut off at August/September, but April/May born boys are routinely being deferred now to not be the youngest) we are getting 20yos high school graduates (which I see as a problem, not as an advantage).
However, together with keeping the general education the whole high school (for the academic track), the students really end up on average 1 and something year older with a full extra year of learning.
I am not saying our system is necessarily better, it has it's disadvantages and problems. My point is that international students are coming at different ages and from different learning paths, and some of them are older.
I would hope that the UK universities are starting at the level expected at UK high schools, so international students having a slight advantage doesn't really matter.