I'm not sure "centre-left" is really what they need to aim for, but they need to look realistic rather than having a manifesto that looked like it had been generated by a "socialist bingo" meme. I know people on left twitter have a massive hate-boner for incrementalism, but campaigning on "DO ALL THE THINGS!" just doesn't work. My advice would be to focus on a few (relatively popular) left-wing policies and push that message hard. And make sure that the messaging is in terms that everyone can relate to, rather than the language of woke-twitter, politics students and the "metropolitan elite".
As specific examples of policy, Labour need to drop trying to nationalise everything at once since no one believes they can do it or pay for it (it doesn't even matter if they can, if people don't believe that they can). If you aren't an actual socialist (i.e. the vast majority of the UK voting public) then the benefits of nationalisation versus privatisation will not be immediately obvious. It just sounds like abstract or meaningless political ideology.
Pick one industry that needs nationalisation (water or trains seem like the easiest to argue for), work out a plan for how to do it and explain exactly what nationalisation will do to improve services for the average person. Do not make this a flagship policy. If Labour win, they do their plan and show that it worked, then use this to argue for more nationalisation.
Instead we got "free gigabit broadband for all" as the only policy anyone remembers from the manifesto, which is probably the most out-of-touch policy you can imagine.
For the other failing privatised monopoly industries, just push a tough-talking message that you won't be using taxpayer's money to bail them out every time they run into trouble. If/when they do run into trouble and need a bailout, you can use it to justify nationalisation in the future.
A lot of why Labour lost was due to messaging. Massive radical socialist changes were not credible and the scattershot policy announcements meant that they were unable to counter Boris's simple claims to "Get Brexit Done" and "Build more hospitals". Labour's most popular policy was to invest in the NHS, but they expressed it in purely anti-American/anti-capitalist language and vague claims that the Tories would destroy the NHS. Meanwhile Boris claimed he would increase NHS spending and build 40 hospitals. It was a lie but it was a concrete promise to do a thing with clear benefits to normal people. "More hospitals" provides a clear benefit, while "stop Boris selling the NHS to the Americans" requires a lot of understanding of how the NHS operates, how it can be 'sold' and the many complex ways that American trade deals will undermine the provision of services.
A lot of the problem was that while we know Boris lies, most people also think Corbyn lies, that all politicians are liars and that they might as well vote for the liar promising more hospitals because he'll have to at least build a few after promising to build 40, right? (oh my sweet summer child) Similarly, the "Actually the Tories are only adding 31,000 more nurses and lying about 50,000" arguments were a complete failiure, since people just thought "Well, 31,000 is still a lot of nurses - have Labour even said how many they'd hire?"
