We'll have to wait until the next manifesto but just going off the past history of the "centre left", they don't take it seriously at all. Yes they believe it's happening which is better than the other guys but they aren't interested in challenging the causes of climate change(Because that would require fundamental changes to capitalism). Yes it's better to acknowledge the ship has hit the ice berg but that's not going to stop the ship from sinking.
As for what to do or just sitting out, I honestly don't know. The only reason we had decent social democracy in the Labour party was because the centre left lost in 2015(I voted for "red ed") but more importantly covid has basically proven the socialist critique of neoliberlism to be correct but also shown socialists to be massively out of date.
Even if we pretend and say Starmer runs on the 2019 manifesto and then somehow wins(Putting aside the number of both state and international institutions that would stop any policy), it's still not enough. National universal healthcare isn't enough there's has to something more, national plans on climate change isn't enough, even giant members states like the EU or NATO will be roadblocks to any real change, etc etc. We need something towards a transformation of our political economy
Granted it's a bit on "brand" but there's needs to communist movement but you can't just will these things into happening, sadly.