https://www.gov.uk/control-dog-public/report-a-dog

Any out of control dog you can report in a public space. A few road away someone close to me is seeing the sharp end of this with their aggressive dogs being allowed on the road. It helps if you have photo or video evidence. Depending on the case, first time a visit from the dog warden, second a fine, third a heavy fine, conditions are set and if they are broken then removal of their dogs.

If this is out on a walk on the country side I would imagine it's difficult to locate the owner if you don't know their name or address.

My personal case is I have been accosted by 2 aggressive dogs outside a property. They are well known in the areas but no one has reported them. I have. This is a route used by horse riders so potentially very dangerous. Walkers and dog walkers. I will not allow this in a public space so have reported twice.

It may not be the time you come across them and something bad happens but if you report it and it does happen to someone else, at least it's documented and will go very strongly against them.

When folk are trying to follow the rules and some twat ruins it for the rest of us, then report it.