You've touched on what has been a very contentious question in the historiography of Britain, which centres around the notion of a British 'decline'. Declinist histories, which became popular in the 1980s, argued that Britain had been performing poorly throughout the 20th century, and that it was only after Thatcher came to power that things began to change. Anti-declinist histories soon became very common as well, in response to the popularity of declinism.

Basically, it's arguably still not a settled question, and depends on what aspects of the British economy and industry you decide to focus on (and, I would suggest, which part of the political spectrum you hail from!). I would argue that the foundation for the idea of decline (and later revival) is not, as you say, stagnation and recession, but that British business, particularly the manufacturing industries, had experienced a deterioration in productivity, with knock-on consequences for the economy.

Even narrowing the scope of the argument to manufacturing productivity doesn't really settle the question, as different authors will use different measures of productivity, will make comparisons with different countries, and so on.

Another important point to remember is that declinism isn't just a historiographical concept - it has been part of the way Britain, and British politics, thinks about Britain throughout the 20th century. It has been argued that declinism isn't just one immutable concept, but actually shifts meanings dependent on the time - pre-war declinism was different to post-war, and there have even been different types of post-war declinism, including the productivity one I mentioned earlier.

Along these lines, it has been argued that the rise of economic statistics - a post-war phenomenon - meant more ways to measure (and argue about!) different aspects of the economy, including industrial productivity. Depending on what platform politicians in the UK were running on, they would use different statistical measures to highlight decline in different parts of the British economy, despite however flawed those statistics or arguments might be.

A consequence of this is that politicians, and popular thinking about Britain, has been bound to the notion of decline, because once you create a problem, it's very hard to make it go away again - especially if the other side keep moving the goal posts and changing which measure they're using to beat you with.

I haven't yet directly answered your question about whether or not the causes of British stagnation and recession were long-term or different throughout, and that's partly because I'm not an economist, and not even an economic historian - I won't pretend on that front. However, I have situated my argument in the notion of 'decline' to show you that the premise of your argument may not be as fixed as you believe.

I will leave you with one account which shows the contentiousness of your premise. Within the anti-declinist corpus, I have in particular come across one argument which suggests that the notorious poor economic performance of the early-mid 1970s came about as a result of short-term contingent factors, such as the OPEC crisis, and that the British economy was in fact in recovery by 1977-8, before Thatcher's economic liberalism. This author goes on to argue that it was only politically, but not economically, important events in the later 1970s, such as the Winter of Discontent, which served to distort this narrative into one of Labour's failing economic policies throughout the 70s, with revival only occurring once Thatcher's economic liberalism had been implemented.

Finally, I will subvert the discussion entirely and point you in the direction of "post-declinist" literature. This literature subverts the idea of what it means to have a "good" economy. I will use broad strokes here - the similar arguments which underpin these accounts are that "growth" and a "strong economy" were post-war answers to how to improve the well-being of a nation/its population. Post-declinist authors would argue that growth is in fact a misguided way to improve wellbeing, and that the quest for growth has perhaps done more harm than good.

I hope that helps with your question - let me know if you have any more - I almost wrote a lot more on anti-declinism - and I will try to help!

References

Some Declinist histories

Coates, David. The Question of UK Decline: The Economy, State and Society

Elbaum, Bernard, and William Lazonick. “The Decline of the British Economy: An Institutional Perspective.”

Wiener, Martin J. English Culture and the Decline of the Industrial Spirit, 1850-1980.

Productivity as the foundation for declinism

Tomlinson, Jim. “Not ‘Decline and Revival’: An Alternative Narrative on British Post-War Productivity.” In Business in Britain in the Twentieth Century: Decline and Renaissance?, edited by Richard Coopey and Peter Lyth.

Declinism as mutable and related to economic statistics

Tomlinson, Jim. “Inventing ‘Decline’: The Falling behind of the British Economy in the Postwar Years.”

The 1970s as a result of contingent factors, as opposed to long-term

Tomlinson, Jim. “Not ‘Decline and Revival’: An Alternative Narrative on British Post-War Productivity.”

Some post-declinist literature

Offer, Avner. The Challenge of Affluence: Self-Control and Well-Being in the United States and Britain since 1950

Layard, Richard. Happiness: Lessons from a New Science

Wilkinson, Richard, and Kate Pickett. The Spirit Level: Why Equality Is Better for Everyone

I dispute your assertion that there was any serious pattern of stagnation or recession throughout that whole period. Here is a quick graph of the UK's economic growth rate since world war 2, and as you can see, barring short-term peaks and troughs, it's remained roughly constant throughout, with only the first period of negative growth preceding Thatcher's term of office.

The economic woes that preceded the Thatcher premiership relate more to the immediately prior years, when there was a period of very high inflation (generally the 'opposite' symptom to low economic growth) and rising unemployment (to the extent that the prior Labour government needed an IMF bailout) coupled with high levels of industrial action. When Thatcher came to power, the economy almost immediately went into recession, which meant that inflation was significantly lower, while unemployment skyrocketed. It's only her record after the early 1980s recession subsided that is generally hailed as the Thatcherite economic success story, such as it was.

As no one has really tried yet to answer OP's original question: "WHY did the British Economy suffer throughout the post war period until the rise of Thatcher's economic liberalism?", I will take a stab at it.

Hypothesis: The British economy suffered (relative to other economies, not absolutely) because of increasing Socialist Policies, and nationalisation and central planning throughout the post war period until the rise of Thatcher.

It seems (from many examples) that Socialist policies and a centrally planned economy are not optimal for long term economic growth.

A good example would be how growth in China was constrained under the Central Planning regime of the Communist Party, but took off once the Party introduced "economic liberalism".

A similar (though less dramatic) series of events seems to be exactly what happened in Britain.