Perhaps a look at Birmingham City Council is instructive.
2019 Decide to take IT back in house.
Switch from heaviliy bespoked SAP to Oracle.
Current estimate £100m
Migrating a single major ERP to another single ERP.
This top-down centrally planned migrate-and-merge has f**ked up so many times before what's the odds HMG wil succeed? 100:1?
Don't get me wrong. Data centres should be merged to the places with the lowest operating costs. Remote working staff should not have to migrate if that's somewhere remote (next to a hydrolectric dam in Cumbria for example).
But it should be organic, encouraging mergers, not forcing them.
Of course following the successful campaign by SI's to emasculate the Civil Service of not just people who can do technology but even people who can manage the process of doing technology they have no f**king clue of what a s**tstorm this will be.