Well, a LOUD noise - like from a compressed gas pocketable can 'air horn' may be a momentary deterrent. They can certainly be attention getters. At least you may gain a witness or two...
We live on the outskirts of Birmingham. My wife thought my daily decision of which to pocket carry - an Airlite .44 Special or .38 Special - was unnecessary. That changed last week when a 20yr old college student, working part-time with her, was car-jacked at a four-way stop on the way home from work. The two attackers forced him out of the car and one attempted to execute him with a head shot from a small pistol. He struggled, saving his life, but incurring four gushot wounds; neck, arm, stomach, and thigh. He was blessed - no vital organ/nerve damage - no sepsis - he will make a full recovery. Within the last few years, there was a home invasion that cost the elderly couple their lives - and four Mexicans were shot - executed - by three black competitors in the local drug trade. This was all within two miles of my home - in a once very quiet residential suburb of Birmingham - Alabama, of course.
Fortunately, it is relatively easy to get a 'carry' permit here. Even a Canadian citizen friend legally bought a .45 - and got the sheriff to issue him a carry permit. My wife and I just got back from our pre-dawn walk. I felt naked - only a knife, ID, and flashlight (The gym short pockets are too-small for a .38!). Still, what if I was in NY or NJ... much less, England??
Stainz