DOR will not have the experience or expertise to compete with the big corporates, because any staff at DOR who are competent will be poached very quickly. I couldn't see DOR winning a single competitive franchise tender, so what is the point?
DOR as is cannot bid because it is part of DfT.
To bid, some other government body will have to sponsor it and provide the deep pockets necessary.
I can't see which central/local government body would be interested in doing that - they have other priorities for their limited funds.
SNCF/DB/NS all have rail operations as going concerns with all the skills and financial backup for a UK bid (bank loans etc).
There is no domestic equivalent in the UK because it's all been privatised to the TOCs.
DOR managers will probably end up either as consultants to DfT, or be poached by the TOCs.
At the edges, I suppose publicly owned TfL might bid for LO or Crossrail one day as an alternative to the current concession format.
But they are not risk-bearing franchises like EC and the rest.