I do hope that this post won't offend you as it is only written to offer some advice in relation to the situation you have described.
The most important piece of information regarding this outbreak comes right at the end of your post, where you described a previous incident that has scarred you and caused a fear of bull terriers. Dog's pick up on feelings very quickly without you having to touch them, you only have to be in the their space. When you realized what kind of dog you were coming close to, your fear would have been picked up by your own dog causing him to react in the way that he did... he would be protecting you because you were afraid. The bull terrier was reacting to a dog attacking him, acting out and showing aggression towards him. You say that both were unharmed, that to me says that the bull terrier was just correcting the bad behaviour your dog was demonstrating... you've seen what a bull terrier is capable of first hand so you know what I'm talking about.
I don't know if you can see this video, but it may give you some understanding as to how sensitive dogs are to feelings and emotions... especially fear.
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From your first paragraph, it comes across that even though your dog will cease aggressive behaviour... growling etc, on command, it shows that you don't have leadership of your dog... he shouldn't be growling to start with, this needs addressing.
I won't make comments on the fight between people as I wasn't there to witness and have no way of knowing the facts, I'll leave that to others
