Edward 93 has summed it up most accurately.
Nearly everyone in the British Isles has viking or Norman (or some Roman) blood in them to a greater or lesser extent.
Now that's out of the way, you can see that the previous occupants of these islands were Celts who retreated to Scotland and Wales when the Angles and Saxons arrived.
The languages of Wales, Scotland, Ireland and Brittany are related (very rougly - gaelic) whereas English is a Germanic language liberally diluted by Norman French.
Diet has traditionally been a great distinguisher:
Scottish developed the Haggis (and may they be eternally blessed for this, a good one is food fit for the gods) and proper whisky.
Welsh developed lava bread (funny that this hasn't seemed to catch on elsewhere).
England is the land of great beer (except for the 'Boddingtons or Tetley's cream flow' rubbish) and Roast Beef.
There are physical as well as cultural traits. Again, at the broadest level of disctinction:
Welsh - short stature, dark curly hair.
Scottish - highlands - large stature, red hair.
English - brown or fair hair, taller.
Note that the above is an exteremely high level set of distinguishing features. There are millions of variations and contradictions to the theme. You'll need to do a very in-depth study of anthropology to find out more.