Or alternatively why not skip the completely unnecessary nationalism all together (in this case English) and stick with the many strong franchise identities we already have, such as GWR, LNER and Transpennie Express, and replace/do away with the poorly performing ones?
Two reasons.
Firstly: Labour (and future governments) will want to distinguish themselves from poorly performing TOCs. So whilst I personally like GWR and Transpennine branding, their public perception is less than great. LNER comes away better, but so I think does Greater Anglia.
Secondly: The fewer differing liveries and branding variations the better. Partly to keep costs down and for greater rolling stock flexibility, partly due to this creating a more unified, coherent identity for passengers. Rail England or something like that would be the most accurate description, seeing trains will operate mostly in England.
I quite like the Swiss (SBB) approach to this. They have one national livery, but still divide their train services into EC, IC, IR, RE and S-Bahn. Branding is not synonymous with livery. The service branding is used prominently on maps, advertisements, in the journey app and on PIS displays. The system is easy to navigate due to the distinct number and colour-coded train service brands, but not overly confusing, because there is still a single livery and "corporate" identity nationwide (all trains and stations). Ticketing and journey planning are fully integrated.
The less visible benefit is flexibility. I actually often see the flexibility created through a single livery in action where I live. In the evenings, IR and RE rolling stock is used to operate one of the IC routes (IC8) and I have often spotted RE or IR double decker trains being used on the S-Bahn networks too. Not to mention the use of replacement or reserve trains. This is something SBB does frequently to counteract knock-on delays. They will often have trains waiting on standby to take over a certain service in case of breakdowns or major disruptions, particularly for delayed services coming into Switzerland from Germany or Italy. So flexibility matters.
Their service types translate to the following:
EC = EuroCity: cross-border InterCity services
IC = InterCity: express services between cities
IR = InterRegio - most often duplicate InterCity routes with more frequent stops
RE = RegioExpress - create additional regional rail links filling in gaps, or duplicate a S-Bahn service with fewer stops
S-Bahn = commuter rail services in and around major cities - usually heavily influenced by local authorities (the respective canton)
All service types are numbered, by the way and there is a map with all long distance (EC, IC, IR, RE) services and of course regional maps with S-Bahn services. I think GBR could learn and take a lot from this approach.
"Intercity by GBR"
"Regional by GBR"
"South East by GBR"
Then the regional mayor brands for the local bits.
I quite like this. Using the service types SBB uses one could merge EC and IC into one "IC = InterCity" service type. IR and RE into one "R = Regional" service type. Perhaps one could still differentiate between R (Regional) and RE (Regional Express). Then create "South East" for London and the South East of England specifically and then the locally devolved authorities would decide upon their own scheme (whether that's S-Bahn inspired, commuter rail or whatever else they want to call it).
Alternatively, TfL could develop its own commuter rail scheme covering Greater London, but it may be hard to differentiate London commuter and suburban services from regional and long-distance services in the South East more broadly. Could be a long-term ambition though, I mean there is nothing to say London couldn't have a RER or S-Bahn style system, supplemented with GBR InterCity and Regional rail services. I think that might be easier to navigate than the present system. I think one would have to seriously consider route numbering or lettering though, because the individual line name concept is already starting to reach its limits with LO.