It appears that leash laws are different in the UK? Correct me if I am wrong? People are allowed to have their trained non aggressive dogs off leash, under their control. If I am correct, that means when your dog (the aggressive one) loses it because a "non aggressive" dog gets in his face, you and your dog are at fault even though yours is on leash. Right or wrong?
If that is the case it sounds like you need ways of dealing with dogs that get in your dogs face and I don't know what is allowed in the UK. I keep a whistle around my neck to scare away intruding dogs and coyotes
You may also need to get a muzzle type lead. Again, I don't know your laws. I just did a little research because I was confused about how a person could be mad at you when his dog was the one that was off lead. Sounds like a difficult situation. Wish you the best.