I live in London and I am planning to immigrate to Canada under FSW program.
I believe that the main motive for me to leave the UK is for personal reasons. I have lived in North America (both USA and Canada) and the quality of life is definitely much higher than in the UK. Money gets you there a lot further than it does in England. I also have half of my family living in the USA, but I choose Canada because of its natural resources, low population levels, and fairness in society. Being from UK and having Spanish and Portuguese background, I would love to move to the Mediterranean region, which is easy being a European Union citizen. HOwever, my partner (American) does not speak any other language but English, and, therefore, a move to any other country in Europe would be a step back in my partner's career.
Canada offers in a way a similar culture to that of the UK and France, it is cosmopolitan, and we are not going to lose our European citizenships anyway, so we want to give it a try and bring up our family in a much healthier society, where summers mean going to the lake to swim and winters mean going cross-country skiing.
POSITIVE THINGS ABOUT UK: Cultural things to do are endless, you do feel like you are in the middle of the world, our airports will take you anywhere at low prices, great and interesting job opportunities, british are very open minded people.
NEGAIVE THINGS ABOUT UK: Weather (there are no seasons, summers are mild and winters are mild, it does not rain as much as people believe, but it is cloudy very very often). It is overpopulated (61 million people) and you can tell by traffic, buildings, road sizes, there are lines everywhere. Customer service is non existant, which for me is probably the worst bit if I compare it to many other areas. Everything is extremely expensive. Crime is on the rise.
I hope this gives you an idea as to why some of us may want to move to Canada. The UK, as any other country has very good things and very bad ones. It all depends on what you values most.