In the UK, there are certain situations where our legislation dictates that dogs must be kept on leads. For instance, it is an offence if your dog is not kept on a lead when directed to do so by an authorised officer, so you should always have a lead handy if you are out and about with your dog. Your dog must not chase or attack livestock (cattle, sheep, goats, pigs, horses and poultry) on agricultural land, so keeping him on the lead in any fields of livestock is a good idea too. Dogs must be held on leads on all roads as well (according to the Road Traffic Act 1988) and local authorities will often have similar bylaws covering public areas like parks and communal areas so it’s worth checking for signs before letting your dog off in case there are restrictions in the area you’ve chosen to let him roam free.
Finally it is also an offence to let a dog be dangerously out of control anywhere, which includes public and private places, so ensuring you have good control of your dog without a lead is essential before releasing them anywhere. Regular training in obedience and recall can help to keep your dog safe and under your control even at a distance.