The international trade is just not worth the effort for cheap rifles, but for collectible rifles it is (speaking on collectors, not on large stock). And if you think all the business is done on GB, then you are wrong. I know a gun dealer in my country that has several hundred thousand guns in stock. Do you know any gun dealer in the US that has this stock? In certain countries it is easier to import and export firearms than in others. I've imported firearms from US, Canada, Norway, Scotland, England, Australia, Switzerland, South Africa, France, Germany, Bulgaria, Romania, Finland and New Zealand (plus surely a few more I forgot upon). It isn't much of a trouble if you become familiar with it.
Re Polish guns, bad sample. The Polish used to drive up the European market. A few years ago a Polish VIS Radom was € 1000 in Europe. Now you won't get one under € 4000 with very good samples all over € 5000. Heck, they even paid the € 5000 for a Polish VIS Radom holster. For the moment the import of a single Polish VIS Radom from the US to Poland might make this particular gun too expensive, but the time might come when the difference in value is such large that despite the paperwork and costs is so large, that an import is attractive.