Greenkouki has underlined one of the biggest hurdles. Although I should say you might be able to find working lines from eastern europe that are c/d, the concern is health. While all lines of dobermans are inherently at risk of DCM and there is no such thing as a DCM pedigree, some lines seem to be more impacted than others, and some more severely than others. (A dog dying of DCM at 11 years old or 13 years old is not the same thing in my eyes as an entire litter dropping dead before the age of 5).
Where I will disagree is the willingness to sell a dog sight unseen around the world. Yes it warrants suspicion but as a general rule European breeders have a different mentality from North American breeders, and that is not necessarily a negative thing. My friend who imported her bitch from Belarus, 3 years later still talks to her breeder and has support from her, and her breeder always is asking for pictures, videos, of the bitch and loves getting updates about her health testing results, her show results and her ongoing training. Also don't forget that FCI doesn't have limited registration and in some countries it is in fact illegal to try and retain breeding rights, such as in France, non-breeding agreements are illegal. They don't have the luxury of protectionism in the same way we do which means they actually have to be choosier with who they end up selling to. On the other hand in order for a dog to be able to breed and have their offspring registered the dog needs to be inspected, approved and admitted to the LoF (using France as an example still).
One concern I'd have as well about the Eastern European dogs is now some breeders are leaving some of the pups undocked/uncropped but then will amputate the tail on request at 8 weeks which comes with a whole other set of issues.
I don't think the majority of American working line breeders are leaving their entire litters undocked and uncropped. A few will no longer crop and dock at all, some are still systematically docking and cropping, and some are flexible or dependent on the litter's plans. More UDC/working line breeders in the US are also attempting to produce correct conformation and type and many are showing in UKC venues as well as UDC and then breeding working lines with showlines with the hopes of producing AKC Chs who can also be drivey performance and protection sport dogs. Not all of them are on the UDC breeder listing either
I do agree with GK that if you're considering importing a working dog from the US or Canada, chances are very high you're either gonna have to develop a long term relationship with a breeder before you even purchase a pup, or you're going to have to come pick up the pup yourself so that they can shake your hand and look you in the eye.
From your point of view though I'd try and find out if it is legal for a cropped and docked dog to compete at IPO/IGP venues in the UK, before you go about seeking a high drive working line dog. Some countries no longer allow them to, even if the dog was imported. The exceptions I have seen were geared at foreign handlers who were making the trip from outside the country, and this is no longer the case for conformation showing. It would be a shame to go through all that trouble only to be unable to compete (unless you are just happy training the dog at the club with no competitive objectives).
I like your username by the way.