The major flaw here is that Poles have been in the UK in significant numbers since the war and pre-EU. OK so there are now many more and whole shops have sprung up but they are here for the duration, now into second or third generation. So you won't be short of cabbage soup after Brexit.
Yes there were Polish communities in the UK, Bristol, where lived for over 20 years has had one since before WWII, there was nothing like the impact on culture that there is now. There may have been a Polish deli somewhere in Montpelier, but you didn't have half an aisle of Polish, and other Eastern European delicacies, in most supermarkets throughout the country.
A number of Poles who have come to the UK may choose to remain here after Brexit, and may be allowed to do so. Many or most will choose to return to Poland or go elsewhere in the EU. If it follows the same pattern as the Polish community prior to the EU influx, in 40 years there will be a number of old Poles talking of the old country, but the second and third generation Poles won't keep the old traditions going and gradually those Eastern European delicacies will disappear from the shelves.