Yes it is for all as your dog has to be tested to ensure that it is suitable to do PAT dog work.
Sorry not sure about their website address but I'm sure that if you go through Google you will find it
With you saying that your dog is still nipping i presume it's still a very young pup? I think that they have to be at least 6 months before they can be a PAT dog, could be slightly older.
It's well worth it and can be a lot of fun but you have to be on your guard all the time as some people don't like dogs and one woman who acted as though she was going to stroke one of my dogs hit her on the head right next to the eye, she got glaucoma 2 weeks later in that eye, she already had it in the other eye but there were no signs of it developing when she was seen just before this incident at the veets and was put down soon after as she couldn't cope with not being able to see.
Go on sign up, it doesn't cost much and you really can get a lot out of it as can your dog, but don't do too long a visits at the start as it can be quite draining for your dog.