It wasn't a matter of quality, Diana, it was etiquette. You didn't support the opposition - it was like scoring an 'own goal' in football, and you would be a marked man (sorry about the sexism there).
In them days, Ansells had an excellent reputation for its beer and for most of its pubs. I remember the last steam lorries (drays) brought out of storage during WW2, when petrol was short. The advertising slogan was "Ansells- the Better Beer".
Since then both the big breweries have disappeared, though the title 'M&B' has been raised from the dead for the rump estate of managed pubs that Bass/Charrington/Worthington/M&B (to name a few) ran once upon a time, and owing to politicians' intereference, they don't brew any more.
If anything, I think Ansells' history is even sadder. They were taken over by Allied breweries, who owned a large number of breweries. They sold off the Aston Cross brewery to make room for the Aston Expressway, possibly not their fault, although they seemed quite happy to lower standards as their other breweries replaced the Aston. In particular I don't forgive them for setting up a distribution depot in Holford Drive, Perry Barr and calling it the head office of Ansells Brewery.