Well behaved dogs are permitted, except if noted otherwise in the walk description. Owners are responsible for the control and cleanliness of their dogs. The walk is likely to pass through farmland at some point, and where there is livestock the dog should be kept on a short lead and under control. If your dog is small enough, pick it up when crossing a field of livestock.
The walk leader may ask you and your dog to wait at the entrance of the field while he/she leads the rest of the group across the field and then comes back for you. This allows other members of the group to cross the field before the livestock notices the dog.
Only if things have gone really wrong and you are under direct threat from an animal, should you let your dog run free. This action may save you from getting hurt and your dog can probably out-run the cattle, but letting your dog run free when there isn’t an emergency may well create one!