How do you know that the British imports truly bring new genes?

The Jack Russell isn't a Kennel Club registered breed so a British import cannot have an official KC export pedigree. Those breeders who truly breed working terriers do keep notes on which dogs they use so many of the imports have unofficial pedigrees.

The Finnish geneticist Hannes Lohi http://www.koirangeenit.fi and his team has agreed to make a study of our imports to determine by DNA analysis how the British dogs (group 1) are related to each other, how much they are related to the other group(s) and to the common Finnish lines. This enables us to get true information about the genetic base of our project.

This is of even greater use than are pedigrees, because often pedigrees don't show enough generations to reveal the true historic inbreeding in a breed. If for instance a breed has evolved from only a handful of dogs, but there are over twenty generations, a normal 4-10 generation pedigree can look almost inbreeding free even though the dog really has quite a high inbreeding coefficient.