https://www.theguardian.com/business/article/2024/jun/04/uk-faces-33bn-hole-in-finances-or-return-to-austerity-thinktank-says
Labour and the Conservatives want to reduce national debt, but Resolution Foundation says promises ‘detached from reality’
it's right wing bullshit.
The UK is never limited in the amount of £ it can issue to cover govt deficits. Read a textbook. Issuers of currency do not borrow i that currecy. Debt and deficits have economic impacts, but there is no hard limit.
The only reason for austerity is that rich twats want austerity so they can continue their insatiable greed.
The consideration should be the cost of debt servicing relative to the size of the economy or the state budget, not some arbirtrary number auch as debt size as percent of GDP. As you say, what matters is the effect. If it's cheap to borrow money, and you're spending that money on something such as infrastructure investment with a positive ROI and a good multiplier effect, then it makes sense to borrow.