From a legal standpoint in the UK, you should technically not put your dog in a harness instead of a collar. The law states that every dog in a public space must be wearing a collar with an ID tag, and whilst it’s not a law that is enforced often, it was made to help identify missing pets more easily.
Saying that, as long as your pooch is wearing their collar and ID tag, you can choose to walk them with a harness if you prefer.
There are lots of different things to consider when weighing up the pros and cons of a harness vs. collar. If you have a large or giant breed dog that can pull, then walking them in a collar can be dangerous. Instead, you could look at a style of harness that will give you a little more control when they do pull.
A harness like the Red Dingo Padded Adventure Dog Harness has a clip on both the back and the front, so you can attach a double-ended lead to it for more control. If they pull too hard, the attachment on the front of their chest will turn them in towards you, preventing their forward momentum. The Halti Black Training Lead is a great option, as it can also be used to connect to one point of contact while you adjust the length of the lead with the other clip.
If you have a well-trained pup that doesn’t pull on the lead and so won’t risk damaging their neck, collars can sometimes be much easier to use. They’re quick to attach a lead to, you only need to unclip them when they get home, and they don’t soak up every bit of moisture and mud on walks like harnesses do. If your four-legged companion spends most of their time off the lead, then a collar is less likely to snag or get caught somewhere.
Harnesses are generally better for your dog’s wellbeing; however, some Houdini hounds can slither out of them if they learn how to. Both options are valid walking tools, but it generally comes down to what your preference is.