Most parks in the UK are off-leash, which I think is actually healthier for dogs in it allows them better exercise and teaches them to socialise with other dogs and people off-leash. Or course, there are scary incidents with off-leash dog attacks, between dogs as well as dogs biting people, but that is a relatively small percentage vs the majority of dogs which manage very well in parks off-lead since dogs easily become used to roaming free and socialising with strange dogs they meet.
Saying that, I'm always mindful of bully type dogs or suspicious looking owners with dangerous looking dogs and try to stay clear of them. After one of my cockers was attacked by 2 stray pitt bulls a few years ago, I also tried carrying around a small fire extinguisher. However, that became very heavy, and I worried about the chemicals in the extinguisher hurting my own dogs, so now I just carry around a bottle of bitch spray (using tear gas is illegal in the UK) and extra loop leads in case the other dog is off-lead with no collar. The thing is, dog attacks can happen so suddenly and quickly, it's really hard to prepare against despite all one's well-laid plans and preparations. I just try to remember to keep calm, as screaming or shouting can actually make the situation worse. Also, if a dog clamps down on another and won't let go, one must be careful about pulling them apart as this can cause ripping and more damage than if the biting dog can somehow be persuaded or forced to just let go. In my case, I had a couple of good samaritans who came by and hit the more aggressive attacking pitt bull on the head with a branch to force it to let go of my poor dog's head then sat on the dog, upon which I picked her up and ran as far away as possible with her. So, carrying a stick may be a good idea too.