The map outlines the % of those identifying themselves as 'Scottish and British only' and 'British only' in relation to each Scottish Parliamentary constituency based on the 2011 Scottish census.
The saturation of red outlines the proportion of those identifying themselves as British. Below is a legend showing the % British each colour represents:

  Below 24%
  24-26%
  26-28%
  28-30%
  30-32%
  32-34%
  34-36%
  36-38%
  Above 38%
The original description page was here. All following user names refer to en.wikipedia.
Brythones