Hmmm, are you trying to scare us off and keep that patch to yourself?
As has been said, don't act like a victim and you'll deter all but the most determined assailant before they even get to you with your body language. Failing that, I find re-adjusting my attire, tucking my shirt in, thereby momentarily revealing my Kabar ("it's for digging officer, naturally, what else?") will do the trick, even deters people who are approaching without malice! That's a last resort though and I certainly wouldn't whip it out on anyone, it means the situation is out of control by then and anything could happen, generally something very bad for all involved. I sincerely hope I've swerved round the issue, even to the point of retreat, before even thinking that cold steel is my only way out.
Avoidance is the best defence, it doesn't take much common sense to check your surroundings out before you even park up to go detecting. If the do-as-you-likeys are pitched up nearby, perhaps come back next week when they are miles away. Disappointing, yes, to have to change your plans, but better than attracting unwelcome attention.
We all know how intriguing a solitary figure in a field is, to the "general public" he is just as appealing - "What's he doing? What's he found? Why there? Ever found any gold mate? I've always wanted to buy a metal detector..." etc etc. Despite all the camo, you'll be spotted by many people wandering past your search, but it helps to not boldly stride by a pub garden with all your gear, advertising your presence to all and sundry. Be discreet, glasshopper.
There's also the image of the detectorist as, well, um...let's just say "someone who keeps himself to himself"...and maybe wears an anorak a lot. To a bunch of lads who scope in on him, the hapless dweeb in the field might be a source of amusement and fun, which easily gives way to escalating nastiness if allowed. Being aware of people approaching and standing up ready to greet them with a smile, and to field more questions about "all those coins that bloke found down South", long before they actually get to you will help you assess their intent and subconsciously project that you aren't easy prey if they feel like having a go. No-one's going to catch me on my knees, engrossed in a hole, not for the sake of a good look round every now and then.
Pay attention to your surroundings and the local "wildlife" and you'll be fine, wherever you detect (apart from Newcastle, obviously!)