Well, it is not a given that the forecasts are accurate, but the important points are
1) They have been produced by the Govt Dept specifically tasked with managing the exit, together with an independent organisation tasked with delivering honest unbiased figures
2) There aren’t any other authoritative forecasts-they are the only ones we have got
So, only a govt of particular stupidity would pursue any option other than the best. Equally, only a bunch of selfish myopic zealots would argue for the worst options at the expense of the best.
Yes you have guessed-we are led by a govt of particular stupidity controlled by a bunch of ....