Just for the record, Scotrail and Transport for Wales are not "already nationalised" in a railway sense, separate from DfT TOCs.
They are still subject to the UK-wide 1993 Railways Act which set up franchising and regulated their periodic re-letting.
Until this is replaced by GBR legislation, the general terms for TOCs still apply to them.
They essentially had an DfT opt-out from reletting to the private sector, and this was confirmed in Louise Haigh's legislation forcing public sector operation of DfT TOCs.
But there are many provisions from the 1993 legislation which still apply to all TOCs (eg ORR regulation), until replaced by as yet unpublished GBR legislation.
This is one reason why GBR cannot be established formally until that legislation is passed.
On top of that, Scotland has considerably more autonomy from Westminster over its railways than does Wales.
Merseyrail, London Overground and Elizabeth Line are also in the Railways Act mix somewhere, with different degrees of autonomy from DfT.