The milk issue was from 1974 when she stopped free milk for older primary school children. Enough time between 74 and 79 for people to choose not to vote for her if they disagreed this strongly enough. Trouble was Harold Wilson's Labour Govt had stopped free milk for secondary school children 4 years earlier but nobody remembers that. As for the miners and unions - they were election pledges, she campaigned for the popular vote on a manifesto to curb union power.
You need to extend your reading back to the 70's to see why we were fed up; it wasn't just the winter of discontent in 1978/79 that was the problem - go right back to the problems in British Leyland, read up on working practices (including sleeping on the job) poor management, shoddy goods produced in what people lovingly called "British Industry" of the time - Austin Allegro for starters) the 3 day working week, going cap in hand to the IMF,