I'm not convinced that there is no real tradition of harmony singing in the UK.

I know Vaughan Williams declared that the English tradition was a single melody line, not harmonised, but he happened to collect most of his songs from individual, mostly elderly people singing solo in their homes. As far as I know he didn't really look into how singing was used in farms, on ships, in factories, at feasts and festivals, or in groups of families and friends at leisure.I agree with Graham above - church music was so much in the blood of many (most?) people in Britain that it would be very surprising if there weren't other harmony traditions like those of the Coppers, whose harmonies are very churchy.

This applies, I think, in Scotland and England but probably less in Ireland; Irish traditional singing often uses ornamentation as an alternative to harmony. This does happen in English and Scots songs too, but there are also an awful lot of big solid melodies and chorus songs that are just crying out for harmonies, and I'm pretty certain they'd have been sung in harmony, especially before the widespread use of pianos, accordions, guitars etc which put in the chords and harmonies for you.

I don't think I can add anything about why we do it, it's all been said so well already. I'd just add that since so many of us feel that it's very natural, almost instinctive, it would be astonishing if it had only just become prevalent in the last generation or two.

Marje