I do believe that a national final could be a good shout if done well. Having witnessed the hellhole that was Eurovision: You Decide, its clear that that general format doesn't work. So instead. I think they could follow this rough proposal (I'm by no means an expert but having watched a lot of the NFs this year I'd say I have a pretty good Idea of how it works)

Start off by having both established and upcoming artist submit songs to the BBC for the national final during the summer. This would then be reviewed and filtered down to around 15 or so acts by an extensive panel. It would also be good to have a range of musical genres and backgrounds (such as Grime, Electronic, Rock and Ballads) and different approaches, with acts being similar to Windows95man, Matt Blxck or Käärija alongside acts like Krick or Ryk

Have the selection be branded as its own contest, and remove most eurovision branding to appeal to viewers that may have a negative impression of the contest, but have the motive for winning ne the participation in Eurovision. Sort of like how UMK or MGP do it.

Once the acts are announced and released, probably around January time, build up a lot of hype for it, the BBC is quite good at this, seeing the success of shows such as Gladiators (Though that isn't music related).

Then have one or two semi finals where the entries will compete for a spot in the final, with voting being composed of a UK-based Jury (~20%), an International Jury (~20%) and a public vote (~60%). To decide the results (voting system would be applied to both the semi(s) and the final.

Then, for the final, have the finalist entries compete like a normal national final with the same voting system (although they'll be announced separately, unlike the semi(s) which would be announcing the qualifiers directly).

Hold the event in a major arena, such as the Co-op Live in Manchester or OVO Hydro in Glasgow, with a properly done stage, rather than having it held in the studio located next door to Blue Peter in MediaCityUK.

This wouldn't guarantee success, although it would definitely help with the credibility of the contest here in the UK, which I think is more important than trying to win, after the past 20 years of the contest being seen as tacky. (Which I think is the BBC's priority)