Flint Ironstag: Who would need a union? What could they offer their members? And what could they threaten their members with if they left?

It would depend on the time and the place, and on unions being powerful enough to control the availability of jobs. Bear with me a little, it's not easy to explain without a lot of context. Britain in the 1960s, 1970s and early 1980s was a very strange place that's hard to picture if you didn't grow up there.

In Britain when Thatcher came to power, the big unions were strong enough to control the supply of labour (sic). For example, if you wanted to work as a miner you had to be a member of the National Union of Miners (NUM). Nearly all the mines were controlled by the government itself (what was called a "nationalized industry"), but even the very few private mines wouldn't hire non-union workers. If anybody did so, the union would go out on strike, not just at that mine but across the industry. The transport unions (who controlled road and rail freight) and the steel workers would come out "in sympathy". Within a couple of weeks, stocks of coal would be exhausted and the country would grind to a halt. This was the chokehold the unions had. Furthermore, the unions at that time were led by a generation of deeply militant leaders (for example, Arthur Scargill) who were so ideologically committed to their beliefs that they were willing to let the country go to ruin rather than concede this control (ask any British person of a certain age about the "three day week"...).

So now imagine that you're a miner. You live in a mining town, where nearly all the jobs -- and certainly all the well-paid jobs -- are down the mine. You're the son and grandson of miners who've lived in the same town for generations. Your only skill is mining. There's no "better paying" mining job because the wages are agreed between British Coal -- the only employer -- and the National Union of Miners -- the only supplier of miners. There's no other job to walk into because all the mines are run by British Coal, not only in your town but in every town. Even if it were possible, it would never occur to you to get a job in a different line of work. You have (or perceive yourself to have) no mobility. We're not talking about people with transferable, white collar skills. From your point of view as a miner, the union owned your ability to work, full employment or not. (And in return, it guaranteed that you got paid, even if the economy was in recession and demand for coal was down and in a normal market-driven economy, employers would be slowing production and laying off workers).

(If you didn't experience it, it's hard to imagine the degree of control that the unions had at the time. The closest American circumstance I can think of is the days when some US unions were controlled by the mob -- except that everything the British unions were doing was completely legal. In many ways it was analogous to the evils of the "company town" in the US -- with the bizarre twist that the exploiter was the Union that supposedly represented the workers, rather than the employer. The Union had in effect become the employer, and power had moved one step along the chain.)

As bizarre as it sounds, this was the reality of Britain from the 1960s into the early 1980s. The big unions really were able to control access to employment so tightly that they didn't have to fear full employment.

For this reason, there is a good case to be made that Thatcher deliberately embraced high unemployment because in those circumstances it weakened the unions. It reduced their membership, and made people more willing to defy the union and take a non-union job because jobs were so hard to come by. Having broken the union's power with the combination of a deep recession, changes to the law, and frankly brutal police action, she was able to return to full employment in pretty much the situation that you described, where full employment no longer handed power to the unions because their monopoly was broken.

So this really is the core of the disagreement about Thatcher's legacy. Did the power of the unions need to be broken? Yes. Did it have to be done at such cost to ordinary workers? I think not, but there's no way to ever know.

Does that help at all?

He does repeat the myth that Margaret Thatcher "declared war on the unions".  In the UK in the seventies and early eighties the unions were open about them using their power to bring down governments or force them to do their bidding no matter what the country at large wanted. They declared war first, and openly. Maggie was the first leader to have the balls to stand up to them.
She didn't declare war. It was self defence, and the UK benefited hugely from her time.
From being totally written off as a country in the seventies to having an economy so strong that when Blair came to power he kept to the spending plans of Major and coasted on the economic success of the Conservatives.