I was around in the '70s (at the end of, in beer-drinking terms), and the answers then would have been Harveys, King & Barnes and Fremlins.
Yeah, it was more a comment on how big brewers with their tied houses dominated the landscape in those days whereas now there is such a huge number - and for those of use who hated that gassy keg crap the big conglomerates used to churn out it's great to see, even if I don't like a lot of it (And yes, I appreciate the irony seeing as I love a cold can of Kronie).
I'd agree with your choices, btw although I'd probably swap Fremlins for Gales.