In short, the Left had completely fucked the UK out of the park, it was a basket case. This is a matter of record.
Yes, by 1979 the UK was essentially bankrupt. But to suggest the unions or "the Left" were the cause is questionable (in my opinion). The problems faced by the UK were at least as much a result of global factors as they were a result of any national policy. By 1979 the entire world was in a mess - with the arguable exception of Japan, but that was for very specific reasons unique to them at the time, and the upwards trajectory they were on during the 1970s was far from sustainable as we now know.
The oil shocks of the mid-70s had taken a toll on the global economy. On top of that, the 70s saw the beginning of a huge global economic shift endangering much of the developed world's manufacturing base. This wasn't the result of UK Labour Party policy, it was the result of free-market globalisation, the industrialisation of the so-called "developing" world and largescale systemic unemployment* in the west. The Labour response to this was far from perfect (I'm not a Labour supporter by the way). But I'd honestly argue that it was better than the Tory (slash'n'burn) response.
In reality, by the end of the 1970s the "developed" economies needed to change significantly. They did this by embracing a peculiarly right wing form of quasi-free-market capitalism (via Thatcher and Reagan, but also many others) built on an entirely unsustainable foundation of public asset liquidation and debt creation. My own view is that the time was ripe for a significant change in a very different direction, and that it would have been far preferable. But it wasn't to be, so we'll never know.
The appearance and elevation of Thatcher was entirely understandable of course. She didn't mastermind the neoliberal onslaught, no more than did Reagan. She was just the logical expression of a cultural response to the economic crisis. Unfortunately, just because something is understandable doesn't make it positive. Lung cancer is an entirely understandable biological response to smoking three packs a day. Doesn't make it a desirable one.
And can I also suggest that we put the "Thatcher's economic miracle" myth to bed for once and for all. Her policies were economic lunacy. She just got lucky by coming to power when North Sea Oil production was ramping up. Given the massive windfall it provided, it would have taken a real effort for any government not to see a steady increase in GDP during the period. What's so tragic is how she blew that money on enriching the financial sector and entrenching it right at the heart of the British economy. Her ultimate legacy - like that of neoliberal economics in general - is one of massive debt, an obscenely bloated financial sector that's lost all touch with the real economy, and social inequality on a grand scale.
Subsidising a major employer is a far better use of public money than putting the workers on the dole. You can disagree with that statement and hold up balance sheets to prove I'm wrong... but I've yet to see any balance sheet that properly accounts for the public good, community cohesion and social justice.
Like yourself, I also see many sides to Thatcher. For me though, none of them are good.
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* By "systemic" unemployment, I simply mean that full-employment is actively resisted by the prevailing economic system and can never be achieved without fundamental changes to that system.
