Gun are a greater deterrent than any jitsu I've seen or the modern age. In fact, jitsu in all honesty is an outdated means of self defense.
I disagree. You cannot always carry
a firearm, let alone use/brandish one. Also, I would say that guns are only warranted/useful at times of imminent danger...very extreme situations few people in our society have to deal with much...And I'm not sure about the gun laws in the U.K., but I'm guessing they're more restricted than here.
In my opinion, the greatest deterent is a combination of confidence, humility, and humor...a couple big friends doesn't hurt either.
Apart from that, learning methods on how to negate incoming force and/or applying force yourself (jutsu) is always going to have a use in a society where people still try to hit each other.