Please fix title. The article title is "EU referendum: BBC forecasts UK vote to leave". The count hasn't even finished yet, and it is still possible for the remain vote to win at the current time (albeit unlikely).
It's still technically a forecast, but the BBC has called it and so has everybody else.
Hardly. Britain has voted. The result of the vote is that Britain will leave the EU. The fact that the votes haven't been counted to 100% doesn't make that fact any less the case. Statistics work.
You'd be surprised at how much day to day stuff you consider an "fact" is actually "statistically correct" rather than an absolute.