Under the last Labour government the pound collapsed within the EU – far more than the latest ridiculous Treasury forecasts of possible future falls.
In July 2008 the pound bought $2. By January 2009 it was under $1.40, a fall of 30%.
In July 2007 the pound bought 250 yen. By January 2009 that was down to just 122. It had halved.
Better stewardship of the UK economy and events elsewhere since has brought the pound back up to $1.45 and to 160 yen.
I do not recall the Treasury forecasting the huge falls in sterling we experienced towards the end of the last decade.
If they were unable to forecast the pound accurately then when such a big change was about to happen, why should we believe their latest forecast?