It's a myth, according to Andrew Marr, and I agree with him:
Britain suffered a crisis in the Seventies, a national nervous breakdown, and has recovered since. Britain in the Forties and Fifties was a damaged and inefficient country which would be overtaken by formerly defeated nations such as France, Germany and Japan.
But the longer story, the bigger picture, is that Britain successfully shifted from being one kind of country, an inefficient imperialist manufacturer struggling to maintain her power, to become a wealthier social democracy, and did this without revolution.
And shift she did, in the greatest scuttle in the world.
Thatcher made all the difference. It may take more time to pass before more Brits appreciate more fully what she achieved.