in the eyes of the law they are not under control if not on a lead
Not strictly true. A dog will be considered to be "under control" if it returns to the owner when called. Obviously not the case for the OP, but I'm wary of taking his account at face value - it sounds like the dog's reaction was due to being startled. But ultimately, yes, it is the dog owner's responsibility to ensure that the dog is under control at all times, whether that's on the lead or off it. It would be simpler to have a blanket rule that dogs should be kept on the lead in public places but as
@mjr says, that's not going to go down well with a nation of dog lovers.
I've always been wary of dogs since an incident when I was a young child, but became a dog owner just over a year ago for the first time, and it has opened my eyes in a big way. I have come to realise that a lot of my problems with dogs are down to my own behaviour towards them. It has certainly changed the way I ride on shared paths when there are dogs around.
Our dog does have a tendency to jump up and bark at cyclists, but that's why we do always keep him on a short lead whenever we're likely to encounter cyclists. And we're trying to train it out of him. More than anything else, I don't want him to get kicked.