Comparing the population of Scotland (and combined Scotland/Wales/Northern Ireland populations) with London
Using QGIS Nødebo and PostGRES / PostGIS, and using 2011 Census data.
The Blue area contains the same population as Scotland (5.3 million).
The Pink area has the same population as Scotland, Wales and Northern Ireland combined (just over 10 million).
The population of Scotland would reach about half-way to the M25
In case you're wondering, the reason why the pink area is much larger than double that of blue, it's because as you go out, population density decreases. This means Scotland's area appears smaller in relation to the combined area than a simple 1:2 ratio would suggest.
The combined population of Scotland, Wales and Northern Ireland would extend just past the M25.
How it was done : Based on cumulative population of output areas, which were sorted by ascending distance from Westminster based on centroids.
QGIS features used : print composer, buffer/dissolve, draw effects/drop shadow, QuickMapServices plugin.