Reeves already has room to boost investment in the near-term, without any changes to the fiscal rules, because she can determine the extent to which headline borrowing- which has to roughly halve over the next five years in order to hit the overriding debt rule - is split between the current budget deficit and net investment.
In the last fiscal year, about 40% of headline borrowing related to the current budget, and the rest was investment. Therefore if she can reduce the current budget deficit faster than currently envisaged, then she can open up room for more investment. So Reeves talking about tough choices, rising investment and no real terms cuts in overall spending is all possible if she has a very strict, austere in fact, control over the current budget. Which obviously fits in with cutting the WFA and tough talk on benefits etc, as well as the spending cuts already announced.
This discussion around tweaking the fiscal rules in order to better capture the benefits of investment, and possibly place these against the rise in debt and allow for more investment, is interesting but fanciful in my view. The big issue here is how to measure these benefits, and markets will rightly be concerned by very favourable methodologies being employed which funnily enough help ensure the fiscal rules aren’t broken.
There may be some movement on this in the budget but it will be very tentative, and any big increases in investment will come from tax rises and very tight controls over other areas of expenditure, and ultimately headline borrowing will still decline. So still fiscal constraint.
Finally, you need to be very careful around this narrative around only the wealthy facing additional taxes, chiefly because it’s nonsense. No doubt some additional tax hikes will be announced and these will hit the wealthiest, but the vast majority of the increased tax revenue over the next few years will come from fiscal drag around income tax allowances, and everyone working will be paying for that.