I think there's a few issues at play:
1 - This is ultimately an election about local councils and from speaking to a few friends of mine working in the public sector there is a feeling that Labour councillors have taken the vote for granted. I have a friend who votes Tory at a council level but Labour at a national level purely because competency trumps ideology at a local level.
2 - Boris Johnson's still a vote winning machine. I think for those who only started paying attention to British politics post 2016 his rise seems fortuitous and intertwined with the Brexit question but he's been winning elections against the odds ever since he nabbed Mayor of London as a Tory. He's ideologically not wedded to any position and in effect his government has almost crafted itself in opposition to Cameron and May's governments. Meanwhile, Sunak in Number 11 plays the role of the classic suit which reassures the old school voter base from getting too nervous.. Labour will need to have a similar sized personality just to be noticed.
3 - By pivoting away from austerity - which the Tory party was beholden to - the attack lines open to Labour have narrowed. On the subject of climate change, public health, economic policy, immigration etc the current government are arguably to the left of the US Democrats When it came to Covid relief, they've been extremely comfortable at printing money into the system without worrying about austerity measures and have raised corporate tax rates from 19% to 25%. Labour have geared themselves to battle Cameron's Tories who championed deeply unpopular austerity for 5 years but don't have a response for a government who've pivoted to the middle ground economically.
4 - Socially Cameron pivoted to the middle. Boris Johnson decided to move to the right. The narrow line between social justice and wokeness is a trap that the Tories lay out and Labour fall into every single time. I do think there's cross pollination of social idealogy between the US Democrats and Labour UK that isn't going down well outside of London.
5 - I think the narrative that all of the North is an industrial wasteland similar to the rust belt in the US is off the mark. Manchester, Leeds and the surrounding areas have received a lot of investment in recent years. Meanwhile, Teeside has seen some serious regeneration - Ben Houchen, a Tory, won his spot with 73% of the votes. Even Sadiq Khan won't get those numbers in London. The Economist covered this quite well a few weeks back: https://www.economist.com/britain/2021/04/03/the-truth-behind-the-tories-northern-strongholds
"But the dilapidated high streets of former industrial towns, which are sometimes compared to the American rustbelt, are only half the story of Mr Johnson’s new domain. For they are often surrounded by gleaming new suburbs: a British counterpart to the American dream, where a couple on a modest income can own a home and two cars and raise a family. “The Tories didn’t win the poorest bits of England,” says a Labour shadow cabinet member. “They took a load of places where, frankly, life is pretty good, and it is more surprising that they were still voting Labour before.”"
"As young professionals priced out of big cities are well aware, Britain does not build enough homes. But some parts of the country have done better than others. The north and the Midlands have accounted for a rising share of housing investment over the past two decades,"
