If you have him outside without a muzzle AND you let him off the lead as well (with or without muzzle) you are breaking the law and failing your dog. This is from the Dangerous Dogs Act:-
"What does ‘dangerously out of control’ mean?
A dog that is dangerously out of control is one that has injured another person, or has given another person reasonable apprehension that it may do so.
This may be something as simple as your dog chasing, barking or jumping up at another person or child if it leads to a complaint.
Consequences for owners may include a prison sentence and a ban on owning dogs.
If your dog is seized, the presumption is that your dog will be destroyed unless you are able to persuade the court that your dog is not a danger to the public.
You may also have to pay a fine, compensation and costs."
Basically if your dog was off the lead,with or without muzzle, and someone was frightened by him, they could call the police and then it would definitely be game over for your dog (and you'd be in the shit as well) as you've proven you break the law.
Your dog doesn't know what the laws are, you do. No excuse.