A new party will face an uphill battle.
It would, of course, be socialist and therefore most of it’s support would come disgruntled labour supporters who are disillusioned with the current labour government. (Keir Starmer is politically around the same position as John Major).
The Labour leadership would see any new socialist party as threat, much as the SNP attack any prospective pro-independence parties. The tories would be happy at first to see the Labour vote split, but that would change if they thought the new party was a threat.
The media are all run by right wingers or “champagne socialists” and would be against it from the very beginning.
And a big threat will come from inside, rather like the current in-fighting in the Scottish Alba party.