@eurocelticyankee,
Well, my Dad was an out and out Cockney, and used to say it frequently.
I don't think it's unusual to hear a Brit say it.
I always understood the literal meaning to be a reference to a batch of wine, possibly ale.
The beer or wine maker would take a taste to see if it was ready for bottling, and if it was a particularly good batch, it would be called a real corker, or something worth shouting about.
The term isn't rare over hare, unless you're under 35, of course. Then one would say "well good", or something equally unfathomable.