Polish cuisine is much like the German variety. Lots of cabbage (sauerkraut is very popular) and they also make much more use of beetroot than we do. Pork features highly and of course lots of potatoes. Polish cooks tend to pickle more vegetables than we do and make greater use of spices.
There have been Polish greengrocers in many places for years. It just seems to have taken them a long time to have come to Berek's attention. I have used them in London and Bedford in the past, like all shopkeepers they stock what their customers will buy.