NJ. Thanks for the link to 'Briefings for Britain'.
Why do you prefer to trust that site rather than the Office for Budget Responsibility? The OBR website states what they are and what they do, and they seem to be a reputable outfit - often mentioned in the news. BforB say nothing about their aims and intentions apart from having prominent 'union jack' patterns on their site.
Sadly, the word 'Britain' has been hi-jacked, as has the UJ, by right-wingers. It's all happened before. In the sixties, Carnaby Street and pop music and fashion enjoyed celebrating Britain and the UJ was prominent. Then racists came along and pinched it all.
And so it goes.
Still, we all seek out reassurance that our views are the right-thinking person's, and so we tend to read what comforts us in our stance.