I think a part of it is because we are still getting taller by generation.. so while the average height for all males in the UK may be 5 9 or so, I would be willing to bet the average height fro males in their early 20s, is higher.
For me, I am exactly 6' and I don't consider myself to be tall enough for it to be of note.. no one has ever come up to me in the UK and said I am tall, or reacted to me as a tall person.. for young people, I would say anything from 5'8 to 6'1 or so, and your just in the 'normal' range, you may be at the high end of normal or the low.. but I don't think people would call you short or tall particularly frequently...
My friends who are 6'3 and above, they are the ones who start to get refferd to as 'the tall guy' 'my tall friend' 'oh my god your tall' etc.