Ashita no Joe is one of those shows that even if you haven't seen it, (and most anime watchers in the west won't have) you've probably seen something that has referenced it. It's like if you've seen the Simpsons, you'll have seen parts of Citizen Kane and Godfather second-hand.
The story follows the boxing career of orphan, Joe Yabuki. That's it. No need to complicate matters, it's a straight ahead sports classic.
The two things you are likely to have seen homaged/parodied/referenced are the numerous training sequences and more obviously – The Cross Counter.
The Cross Counter is where two opponents simultaneously punch/strike one another's face along the other's arm, normally with the result of both protagonists going down. It's a sign of the impact of the series that it's actually added something to the visual language of the medium. According to Anipages Daily, the animated version of this scene was directed by Masami Hata. You have to wonder if the scene would have resonated through the years had it been handled by a less able animator.
Of course an arguably bigger sign of the impact is the fact when one of the characters in the series died, an actual funeral was held for the character outside the publishers.
The first half of the anime (the 1970 series) was directed by the powerhouse that is Osamu Dezaki, a man as I've mentioned before has added a lot to the visual language of anime on his own account.
The age and length of the show means it seems unlikely to get a US/UK release in the current market despite it's position of historical importance. There have been a small number of fansubs made of the series however, though with the last one (episode 3) being done over a year ago, I wouldn't hold your breath seeing it completed anytime soon.