It is usually the act of you running or riding past/ near the dog/s that can cause this type of dog behaviour... dont run, (continuing to run will excite the dog and make it braver ).. stop face the dog/s make yourself as large as possible.. loud noises and shots of water will put off a lot of dogs (air powered horn or a squeezy water bottle !?)....
Chav dogs, like their Chav owners are normally untrained and make a lot of noise (eg empty barrels makes the most noise).
If the dog does go to bite you,it will normally be for your leg/s unless you offer it your arm... Sharp kicks to the exposed underside of the dog will be effective, esp the ribs. also thumb strike/s to the eyes are very effective.... the crack on the nose technique will only annoy the dog further. There are other techniques but further training would be necessary and would take up your running time.
As for the mag-light, hard to prove you carried it to defend yourself... remember you also have the right to use the minimum force required.... a tricky one that.
Avoidance is the best action, cross the road if possible. the actions noted above are used only if necessary... Dont forget to call the Law if attacted or threatened, removal of the dogs from this type of owner can only be good for the dog/s!!
I have been trained and I am a dog lover... who owns a Staffordshire bull terrier, my second.. neither dog has bitten anybody!
Hope that helps?!