I
suspect that the rather 'marmite' "Great British Railways" may be an overarching name that is not often overtly stated at platfom level. The report mentions "national and regional sub-identities" and says that "[the double arrow logo will] increasingly appear on trains, uniforms and publicity material too"; the new signage specifications - which I would expect to have been drawn up with this review in mind - limit the branding to the double arrow. That suggests that, while the double arrow itself will be a strong universal thread, local versions of the identity may be used in practice away from a unified national booking, refund and management system.
Signage we know will be black on white with the double arrow across the whole network:
deliberately bland and timeless to distinguish it from advertising, more legible on dark platforms than white on black. It makes sense to have a national standard for the built environment to reinforce the national nature of the network, rather than the territory of the individual operators, where multiple companies call at the same platforms.
However, I would be rather surprised if any attempt were made to have a completely uniform livery across the trains: there will still be those multiple operators providing the train services, including open access, and I'd expect different liveries for different broad divisions just as there are for the different parts of TfL, which GBR is being likened to. But given the reference to "regional identities" there's no suggestion that we will see the return of a nationwide Intercity livery, for example - though CrossCountry will probably come closest to getting one.
We know the regions will be initially based on the Network Rail regions. With few exceptions, there has been a push towards fairly plain liveries over the last decade, so If I had to put money on it, I'd reckon that each region would have a colour variation of a plain scheme of light base and coloured doors, with some standard shape of block colour at the ends, in colours not dissimilar to the colours in use today on at least one of the companies operating in that area; so green for the (Great?) Western region, lighter green (or blue) for the Southern region etc. There is 'convenient' common ground too: Anglia and LNER sharing red and white in the Eastern region, for example. Will it be exciting? Alas, I fear it will not!
Off the top of my head, ScotRail, EMR and GWR stand out from the crowd with dark bases, as do Southeastern's reliveried trains: I would anyway assume that Wales and Scotland will continue to have their own branding under devolution and the "national identities" clause, but incorporating the double arrow and possibly some of the GBR standards for things like timetables, and perhaps Great Western will be allowed to keep its dark base for a while at least, as it already covers the whole of a defined region.
I would also expect the local passenger travel executives to keep separate localised branding - it would help to define the pay-as-you-go areas - though be required to display the double arrow prominently and possibly being expected to switch to harmonised typography and livery layout.
Adding a double arrow on the carriages would be a simple update. Standardising the typefaces used in publicity and timetables across the network would be fairly easy as most of it is updated twice a year anyway. If something like this happened, at least in the short term, I'd not be completely surprised: