I feel like the article lays it out pretty well? It’s a government who’s been in power too long, whose talent base was massively thinned by a Brexit Purity witch hunt and the impossibility of delivering the Brexit fairy-tale, and who weren’t simply ejected at the last election because of Corbyn’s overwhelming unpopularity.
The article is part of the problem. UK politics and the media narratives around it have beeen utterly wrecked by the fight over Brexit and the attempts to give its creators the boot. For example, take this claim: "Investment is down and inflation higher than it would have been inside the European Union." The UK's inflation in the year to November 2022 was 10.7%, compared to 10.1% in the Eurozone. It just doesn't seem…
I always think that comparing inflation or national debt to other countries is meaningless. A big national debt used for good investment is way better than a small national debt while the infrastructures fall apart, and the state and cost of the infrastructure can be very different in each country. And for the same level of inflation, the effects would be very different based on the resources and market characteristics of the country and which measures are taken.
(edit: and indeed, if you re-read the text around the quote you've extracted, this is indeed the reasoning: the author does not justify that with a comparison of the inflation rate, but with a list of where the Brexit made things worse)