Why do so many british people do have dark hair and eyes?

Yes, that’s true Swedish kids can be indeed quite blonde (Nordic blondness). As with any Northern European population, children start out blonde or just light-haired and progressively usually darken in adulthood. The very same happens with British people.

Dark hair and dark eyes are found in every single European population under the sun, the British should not be exempt from that. One of the main differences, is that the British have more of the dark hair than Scandinavians, Estonians and Latvians. However in eye color, they are comparable especially the Scottish. In terms of skin tone, the British are paler than other Northern Europeans and they tan less as well.

John Terry was a freckled blond boy in his school years and his children have blonde hair.

Scotland is the ginger capital of the world!
http://www.google.com/url?url=http://www.scotlandnow.dailyrecord.co.uk/lifestyle/scotland-proud-ginger-capital-world-7623848&rct=j&frm=1&q=&esrc=s&sa=U&ved=0ahUKEwixgLeIku_LAhVKXB4KHQgaCyUQFggXMAE&usg=AFQjCNF8mbPCPWBUNIxWG8DpEIkHf6IgAg

Palest ("whitest) Europeans.
In order of rank, this is measured not only by skin tone but also skin tanning ability. The weaker the skin tanning ability, the paler or fairer it is.
1.) British Islanders (especially Irish and Scottish)
2.) Scandinavians and then other Northern Europeans (e.g. Estonians)
3.) Central Europeans
4.) Southern Europeans

For blondeheadedness.
1.) Scandinavia, Iceland
2.) Baltic States, Byelorussia
3.) British Isles, the Netherlands, north Germany & Poland, northwest Russia
4.) Belgium, Central Europe
5.) France, the Balkans
6.) Iberia, Italy, Albania
7.) Greece, Turkey

In terms of light eyes, Brits are comparable to other Northern Europeans.

Norway - 55% blue, 23% green, 22% brown
Denmark - 50.9% blue, 33.3% green, 15.8% brown
Britain - 48% blue, 30% green, 22% brown