There are basically three levers that a Government can pull on to try and grow the economy:
Taxation
Goverment Spending
Government Debt
There is a fourth lever regarding regulation which is more of a political stance rather than economic.
Simplistically if you cut tax you incentivise both businesses and workers to grow as they get to keep more of their income. If you increase taxes then you disincentivise both businesses and workers and the economy shrinks.
Government spending- on paper if the Government increases spending then the economy will grow. However the crucial part is where is the money coming from for increased Government spending. If it is higher taxes then the growth from Government spending will be cancelled out by the contraction from higher taxes. And in reality this will be negative as businesses and individuals are far more efficient and productive with their spending than the Government is.
Government debt - increasing debt to fund Government spending produces a boost to the economy in the short term but the debt and the interest acts as a drag on the economy in the long term.
Given the four to five year term of Governments there is a huge temptation to fund increased spending through debt (and thereby boost political popularity) in the short term and then leave the long term debt problem for the next Government. This is where we currently are due to increases in debt by both the previous Conservative Government (with a big chunk of £500 billion on Covid) and also by the Blair / Brown Labour Government.
So what can be done?
Increasing debt is no longer an option (as Liz Truss found out the hard way).
Increasing taxation will slow the economy as Reeves and her NI increase and fiscal drag has found out.
Cutting spending is economically the only way out to fund tax cuts to grow the economy. However politically this is massively unpopular as Reeves and her back bench rebellions has found out.
So the current Labour policy is to cut spending enough to keep Bond holders happy (owners of Government debt) but not too much to upset her back benchers. And to increase taxes enough to cover extra spending but only so that it slows the economy rather than tanks it.
And increase spending to keep the back benchers happy.
In short Keynesian economics is no longer relevant and simply doesn't work given high levels of debt. Economically and to actually grow the economy long term (will cause a contraction in the short term) the solution is to cut Government spending. Politically the solution is to tinker around the edges and continue with the slow managed economic decline.