Pretty much everyone employed in delay attribution would go (no need for the moneygoround anymore).
Management could be streamlined as well.
The 'profits/premiums' leaving the industry would also stop and all money would stay within the industry.
A lot of the GRIP stage rubbish could go as well, if it needs doing (no matter about the BCR etc) then it gets done for the greater good instead of everything having to be cost effective.
Trains could be built up to a standard instead of down to a price meaning things would actually be cheaper in the long run.
You are joking? A "nationalised" railway is not going to be a single body, and "franchises" (short-term TOC contracts) will still exist even if in the public sector.
Think about how costs are central to how the NHS is managed - and how much trouble it causes when they run out of budget.
Railway costs will have to be managed in terms of cost and benefits, and cost control means "delay attribution" and other management tools.
Private sector trains will still run (eg freight, open access, cross-border).
Nobody is going to give you a blank cheque to do what you like, and EU rules and competition law will still have to be followed (eg access regulation, tendering for public contracts).
GRIP is typical of how large organisations, public or private, run major projects, and was invented by central government to manage things like new technology in the NHS, or introducing new aircraft carriers in defence.
It's how you measure the "greater good" and control specifications and costs.
Rolling stock will still need to provide "value for money", just like Networkers did in the BR era, with luxuries trimmed if unaffordable.
You will still have staffing pressures and operating efficiencies to worry about (eg ROCs and DOO).
I can't see nationalisation being the nirvana that its proponents think, and is just ideology rather than a practical solution to the railway's problems (which are all to do with its high cost and subsidy requirement).
All it does is alter the money go round.