I can only speculate from what I would have done in the situation Labour were in. As should be apparent to even the more idea-resistant person (briefly looks at current threads and realises they appear to have gone on holiday) despite the almost softly softly lead up to the election, Labour have been doing a fucktonne under the radar. Presumably with paper and pen, locked away each night and peoples bags being searched on entry and exit. Although given the clusterfuck the Tories descended into at the end, just joined up writing would have been enough.
Part of that process - bearing in mind the shadow Cabinet had access to more than you and I - would have been to triangulate where some money might be hiding. Maybe not precise amounts, but enough of an idea such that by 5pm on 5th July the new government could give the civil service a shopping list of identified projects and say "we want the full fat figures now we are in charge".
This process will continue - it must be mammoth. And as it does, it would be helpful to Labour to keep the public updated with it's findings. Because I don't think the £700million/£10billion on Rwanda is isolated. It may not even be the tip of the iceberg.
With that, the coffers can be boosted with the most pleasant of things. Not unfunded spending a la Truss. But unallocated spend.
And if the prep has gone right. If the Gods are with is, then that will feed into a slowly rising growth chart. And each point on that chart can be invested
I'm not an idiot. Shit happens and things are rarely pure and simple. I am sure U will be disappointed and critical too in the coming years. But nowhere near as angry as I have become at the revolting spectacle of a Tory government shovelling money to it's mates while babies starve to death.