Is this not Dave Braben describing the product as having an offline single player mode? I think it is.
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Therefore does UK consumer law then entitle us to a refund? Yes
Not a student of the law, but I am fairly certain that this reddit AMA from a year ago in which Braben is outlining a planned feature during development that eventually couldn't work out wouldn't count as evidence. I hate to be that guy, but seriously, buyer beware. Next time, don't back KS projects, or buy early access software.
That being said, I do hope this gets resolved to a point where everyone is satisfied. No one wants anyone to lose out, and offline certainly would have been a nice option. That being said, some people are going way overboard with the complaints. No one lied, software development is not as simple as flipping a switch, and sometimes plans and hopes can't make it to reality.
For those who have not played or downloaded, of course, no question, you deserve a refund. But for those that have sunk hours into a the Beta, the loss of this one feature is not enough to justify a refund for a product you have been using for months now especially since the KS promise of a new Elite will soon be fulfilled.