In the end of the problem rejoiners (previously remainers) generally & Labour specifically will have with trying to 'reverse' Brexit is the asymmetry of advantage.
To a large extent the EU has moved on, has other fish to fry & as such is not overly interested in exploring how the UK might rejoin the EU. The EU may be interested in sectional deals & associated trade-offs, but the UK's need for a better relationship is quite one-sided.
Therein lies the problem!
#Brexit
https://www.theguardian.com/politics/article/2024/jun/23/labour-brexit-europe-european-leaders-eu-growth-starmer