It's a "sort of".
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Lower labour costs and/or a better work ethic amongst the non-UK workers, I imagine.
Nobody...?
Well okay, you might be able to argue that large companies have a legal fiduciary responsibility to maximise profits for the shareholders, but meh, I'm not sure that can account for all of it.
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The video may have been tongue in cheek, but it's still pretty accurate in a lot of ways. Pretty much every PM since "Yes, Minister" came out has said how true to life that show is, amusingly / scarily enough.
In addition, you must remember that it's usually a case of picking the lesser of two evils - I really doubt very many people, including those who write them, agree 100% with ANY party's manifesto.
There's also the issue of unexpected things coming up. If you've just been elected but haven't mentioned, say, EU fisheries reform, and then EU fisheries reform is raised at an EU level, the public doesn't really get its say on the matter, because it had no idea it was going to be a topic.