Lots of things have contributed - some government policies, the war in Ukraine, Brexit and - importantly - the Bank of England’s own shortcomings. But the pandemic adding another £400bn of gilts into the APF has been a major factor in the current fiscal squeeze, and this problem isn’t going away anytime soon.
Labour might get lucky again with the economic cycle as they did in 1997, and rates may fall a bit over the next 18 months, but debt servicing costs are going to be a major constraint on what the next government can do with regard to spending. In fact I think it could cause a lot of internal strife for a Labour government and Starmer could have a very brief honeymoon period, as fiscal policy isn’t going to turn around anywhere near as much as some people may hope.