maybe you should try the opposite way and establish an English Shop in Słupsk?
That's a truly terrible idea. There are very very few Brits in Slupsk, when I lived there for a year there were two of us, I know that since then one or two of the language schools there have started employing British native speakers but I'd be very surprised if there were even 20 Brits in Slupsk now. Slupsk is a pretty poor town; yes it is doing better now than it was when I was there but still people don't have much money, and they aren't very adventurous with their money either. Admittedly more than a few people in Slupsk have gone to work in England and some of the ones who have come back might want some things from the UK but they are certain to know somebody who could bring back what they want.
Does anybody have any ideas
Don't even think about it until you are absolutely fluent in Polish and enough money saved to live on without earning a penny for at least two years. Yes it would be nice for your Mrs to see more of her family; however, it is going to be infinitely harder for you to make a living there, almost impossible in fact. Frankly it would be far easier for them to move to the UK.
Tensions with the wife and in-laws escalate, and he goes back home beaten, broke and depressed, and usually separated from his wife and child.
A child who he then sees very little of and pays out a small fortune for due to the Polish family court believing that all foreigners use gold leaf as toilet paper.