Dan did you say you are near Southampton? If so, you should look at training with Pat Tagg:
https://dogtaggs.co.uk She owns german shepherds herself and is very involved in going across to Germany to train them in their original role as herding/flock guardian dogs. She runs tracking training classes and also puppy training in conjunction with another trainer and she uses excellent training methods and is v knowledgeable. You can't go far wrong.
You've also got Natalie Light near you:
https://www.natdogs.com Although she doesn't offer regular classes but more behavioural work if you run into any behavioural problems.
And in Southampton itself, you've got Steve Goodall:
http://www.goodalldogtraining.com Those would be my recommendations.
One final thing to say, is that the socialisation period is extra important for guarding breeds - including GSDs. Getting a puppy at 15wks, you are missing the entire socialisation period (which starts to close at 12wks) and you will be at the mercy of whatever socialisation the breeder exposes your pup to. If the breeder rarely takes the pup off their property, does not expose the pup to many new and friendly people in a positive way, doesn't expose the pup to strange dogs from outside the household, doesn't expose the pup to urban sights and sounds and traffic.... then there is a high risk that the puppy you purchase will be fearful and the main cause of reactivity and aggression in dogs, is fear. (You may not see the fearfulness when you first meet the pup - it often is latent until the end of the first year.)
I say this as a trainer who has many times witnessed people bringing over 15wk old pups - only to see them turn out to be reactive and fearful, usually towards people outside the family. The last time it happened, the people got in touch with me before they'd even brought their "Berger Picard" (french shepherd type dog) over. I warned them. They spoke to the breeder and reassured me the pup was getting socialised... fast forwards to the pup being brought over and of course it ended up being very spooky and reactive.
The dog before that, it was an Entlebucher Mountain Dog. Again, brought over at 15wks, again very spooky and reactive towards people. To the point where I had to advise the owners that a class was too stressful for their dog and not the right environment. They simply went to a less reputable community dog training organisation nearby and persisted in taking the dog despite the fact it reacted multiple times in every class and was clearly very stressed
I have in my classes at the moment, a 17wk old golden who was brought over at 15wks from France. She is completely fine. But goldens are very laid back as a breed, and tend not to be reactive towards people anyway and are probably a breed which can deal with less than perfect socialisation without too many bad effects.
Which is all to say, I would do everything I could to avoid importing a 15wk pup from Europe - *especially* if it were a guarding breed....research shows that the socialisation period closes even earlier for these breeds so it is even harder to expose them to the world in the right way, in time. The only exception would be if I really really trusted the breeder to go above and beyond with the socialisation - which means I know the breeder personally or I have visited their premises or there is loads of evidence of socialisation in a 1-2-1 way on social media.
The reality is that breeders are busy people, they have their own dogs to look after, they are exhausted after raising a litter and they are often keeping a pup for themselves or sometimes even keeping multiple pups on to go abroad at 15wks - which all has an impact on the 1-2-1 time and attention and exposure to new things, which your own puppy would receive. I definitely would not want to import a 15wk old GSD which has been kept kennelled most of its life.