Yaralie - Well that's just a prediction based on a whole lot of guess-work.
Even if true, the economy being 7.7% smaller than it otherwise would have been over 15 years will be hardly noticeable. Historically, since the UK (or any other country for that matter) has not followed the best possible economic policies our actual growth has always been, and always will be, less than the maximum possible. If only we had know what the optimum policies were over the last 100 years the UK economy could have been 100% bigger. I can't say I've noticed any hardship because it isn't? Have you?
To take an actual real-world example, the UK economy is, today, approx. 13% below trend since 2008 (the Great Financial Crash)
linky: www.economicshelp.org/blog/7501/economics/the-great-recession/
graph about 1/2 way down - called output gap.
That 13% (over 10 years) craps all over your potential 7.7% over 15 years. It kicks it in the balls and slaps it's face in the most humiliating way.
Since 2008 have you noticed people dying of starvation in the streets? Has there been much wailing and gnashing of teeth? Have plagues of frogs and locusts stripped the earth barren?
No.
So we have already 'suffered' far worse that your worst prediction and we're all still alive. Not only that we have managed to absorb about 5 million migrants and give them all jobs. We've got record high levels of employment and record low levels of unemployment. In short, we've never had it so good. And that is during a period of 13% growth gap over 10 years (which translates to roughly 19-20% over 15 years. Your scary prediction is only 1/3 as bad as we've already had. I fart in it's general direction.