Good question, and one I've been trying to find an answer to myself since I received my LivingDNA results
Great Britain and Ireland: 95.5%
Scandinavia: 4.5%
I love the Vikings as much as the next guy, but I somehow doubt that 4.5% of my DNA can be traced to some hairy Norseman raping one of my female ancestors during a raid almost a thousand years ago.