I think a more interesting question is why the Scandinavians who share the same rough genetic cline as northern England, Scotland and Ireland (after conquering and colonising them in the early Middle Ages) grow to be noticeably taller. Or a better example might be the Netherlands which effectively shares the same genetic heritages as its neighbouring countries (being the melting pot in the middle) but their population averages taller than either France or Germany. Europe is such a genetic blending that the average population height variances are probably more due to national differences in health and nutrition.
For a poignant insight take a look at the Wiki page showing average heights from around the world and compare the difference between the height of Mexican men (5'4") and American Mexican men (5'7"). That's a three inch difference primarily driven by a modest increase in standard of living!
:) I don't know whether you were just joking, but actually only a very tiny part of the Scandinavian population became Vikings and raided overseas. The vast majority were (relatively) peaceful farmers.
Genetically most Brits & Gaels are different to Scandinavians & Balts.
The paternal DNA know as Y that is genes passed from a father to his son but can not pass via a woman and in turn proves male ancestry. Over years this gene has suffered small mutations which once recorded are placed into haplogroups. The hapolgroup for the majority of the British Isle is R1b. This haplogroup is also common across south and central Europe. This is the gene group of the Megalithic people of central and western Europe from whom the Celts descended.
In Scandinavia they are mostly I or R1a.
I is possibly the highest and is also the Balkans and forms the group IJ. J is the haplogroup of the Chaldeans (Iraquis, Syrians etc) Arabs & Jews) which shows they were related much further back.
R1a is the haplogroup found amongst Slavs, Persians and also NW Indians. It is high in countries such as the UKraine. R1a which was related to R1b much further back is believed to be the genes descended from the nomadic groups of the steppes of Russia such as the Scythians, Getae, Samatians, Persians, Medes etc. R is believed to be the gene passed down from the original Indo-Europeans
Interestingly although Iceland was colonised by Scandinavians it is high in R1b unlike Scandinavia & the Baltic which is relatively low in comparrson.