Bashing London will not help Labour. Or Copeland. Or Stoke.

Shadow chancellor John McDonnell’s insistence on the Today programme that Labour’s defeat by the Conservatives in the Copeland by-election and narrow holding of Stoke against a clown of a Ukip challenger demonstrate something other than the erosion of his party as a national force was as painful as it was predictable. Labour is being reborn as a great social movement, he assured us. Jeremy Corbyn understands our people and will become prime minister, he declared. It was desperate stuff.

The capacity of the Outer Left for self-delusion is as infinite now as it was when I first encountered it the core Corbyn country of North London 35 years ago. But a particular part of McDonnell’s pitch underlined the frailty of his case, or maybe a depressing readiness to offer false hope to voters in the north of England who are turning away from Labour. “We’ve got to rebalance our economy,” he said and described a feeling in the north “that public services are being cut, that London was making all these decisions, in Westminster, and no one is listening to them”.

This is the line McDonnell and Corbyn – both of them London MPs, of course – have been plying for a while, notably at Labour’s economic conference in Liverpool earlier this month. Corbyn pledged to “rebalance our economy” with new transport links in the north. McDonnell promised “at least £10 billion” to that end. “The next Labour government will shift the balance of power and wealth away from a few gilded places in the south-east,” he said.

The idea that some generalised “London” is to blame for problems in other cities and towns was taken up by Gareth Snell, Labour’s successful candidate in Stoke. I’m glad Snell defeated Paul Nuttall, because Ukip are pedlars of prejudice and falsehood. But in pandered to anti-London sentiment, just as Nuttall did, he echoed Corbyn and McDonnell in failing to recognise the true nature of the relationship between London and the rest of the UK.

It is easy to understand why people in Stoke and Copeland and elsewhere think they get a raw deal compared with the capital. The woman on the radio who said “we don’t exist up here” should be respectfully listened to. The trouble is that without London’s economic power, people “up here” who feel neglected by an indifferent London “down there” would be even worse off.

Greater London generates nearly a quarter of the nation’s wealth and around 30% of its taxes. London businesses trade with those in other UK cities, benefiting both. Much of that London tax yield is exported round the country. If McDonnell’s £10 billion investment dream ever came true, London would cough up a big chunk of the cash. A weakened London would be less able to do so. Voters in Stoke and Copeland who want to do London down should be careful what they wish for.

On one point McDonnell did have the beginning of an argument, when he called for more “decision-making at the local level”. But this raised the question of why he and Corbyn have failed to strenuously wrestle the devolution agenda back from the Conservatives, after George Osborne took up the running with his “northern powerhouse” idea.

This spring, Greater Manchester and the Liverpool City Region will almost certainly elect Labour “metro mayors” and the West Midlands might too. Devolution of powers and resources to big cities represents the best chance of those cities and their surrounding areas thriving and making stronger cases for public investment. There will be competition with London for resources, but there will also be co-operation, both politically and economically, which will be good for all concerned.

Corbyn and McDonnell should know better than to feed the politics of resentment. They are no more attractive when directed against London than they are against anything else, and not remotely honest or progressive.