Some updates.
I asked Network Rail for a SVG (a web image format) version of the logo.
They said the logo is now owned by the Great British Rail Transition team ( GBRTT,
https://gbrtt.co.uk/ ), so ask them.
I asked the GBRTT for a version of the logo. Twice.
They haven't replied.
There is an image including the logo on their website, but it uses a different red #e6001e
Aside: Is it just me or is Great British Rail a really annoying name. Sure it can't be plain old BR as we don't want to disrespect our Thatcherite heritage, but even then. Even "UK Transport", or "Transport for Britain" (à la TFL) would be better (and would be inclusive of Northern Ireland).