It is a very interesting question, and probably one that people with a much better understanding of economics and the international marketplace will be a**ble to answer.
However, the story that I have heard of the decline of the British economy goes something like this.
Once, after being one of the first countries to industrialise, we became the 'workshop of the world', supplying and trading goods and resources inside and outside the 'Empire'.
Then, after over-expansion, and two world wars, the country and the economy was left in a much 'weaker' position in the world.
During the sixties, and seventies, many other countries modernised and rebuilt their economies much more thoroughly and successfully than Britain. Trad union and management relationships were disastrous in this country, perhaps due to militance and the relics of an archaic class system that make overt distinctions between 'workers' and 'management'.
As others have said, primary industries (e.g. mining) and secondary industries (e.g. manufacturing) were deliberately neglected, nationalised companies were sold and 'consumerism' and the service economy were prioritised.
The apparent GDP was falsely inflated by vastly inflated property prices, and the minority of super-rich people.
Welfare systems became over-burdened and under-resourced, due to the governments (of whatever shade) no investing in industry, innovation and productivity, but instead following the tenets of 'austerity' and penalising welfare claimants. In a sense, Liz Truss with her mantra of 'Growth, growth growth!' was correct, but the question of how do we grow when we may no longer have the foundations for growth? always remained.
So, the poverty of this country is nothing new. I will not rattle any cages by mentioning leaving the EU, however, leaving the largest trading group in the world, when we have so little to offer, was not a good idea, in my opinion.