Hi Ralph44,
I really feel for you, when you have worked so hard it is upsetting to have all that work undone by other people's unruly dogs. Alas, I think quite a few dogs can also find bulldogs strange/scary and that can also cause them to 'go' at the dog. Not helpful for you though.
I think you do need the help of a good behaviourist, but as Toon says, someone who will actively do training with you. There is a relatively new type of training called Behavioural Adjustment Training (BAT) that was developed for fear aggression). There is a website here. The idea is that the dogs is rewarded for offering a non aggressive behaviour around the stimuli that make them fearful (like sniffing the ground, turning their head) as soon as this happens the dog is immediately walked away from what it fears. Eventually the dog learns that it can control its environment and that it can choose to keep a 'safe' distant from what it fears.
Anyway, have a look at the website
http://functionalrewards.com/ and see if you can find a behaviourist that knows how to do it. I like the idea that the dog is not 'forced' to confront their fears and learns over time that it doesn't have to use aggression to make what it is scared of go away.
Please don't be nervous of the growling. He is saying that he is anxious- all you need to do is acknowledge that he is trying to let his feelings be known- Try to watch what he does before growling, does he freeze, look away, lick his lips? These are the signs that tell you he needs to walk away. Try to react to the very first, tiniest signs and figure out how close he needs to get to another dog before his anxiety levels begin to rise.
Don't try to do this without the help of a proper behaviourist and make sure they know about this form of training.
Do you have a garden and does your dog like to play games with you? Try to play with him away from dogs for now, just to ensure he is getting enough exercise and if he is tired he will be less stressy. You could also try DAP.
Good luck.