Canadian living in London for about 4 years now - of course people are leaving. The pay is incredibly low in major Canadian cities like Vancouver & Toronto; I was looking at 50-60k/yr there, ~150k in the US, or about 60-70k GPB in London. Now, London isn't exactly cheap, and my overall buying power here is probably only mildly superior to what it would be in Canada, but I get to live in a great city with all the amen…
I guess it's partly what people value and partly a novelty effect of appreciating differences when you move overseas but I often see that Brits and other immigrants like Vancouver better than the people who grew up here (though there's an obvious selection effect there) and the same seems to be true in reverse for many Canadians who go to live in the UK.