DO YOU THINK IT WILL BE A VIABLE BUSINESS
Not at all. It sounds like a recipe for disaster. Thousands of people have already had this exact same idea, so you're not bringing anything new or unique to the market.
Your thinking about courier fees is erroneous, as you ignore the costs that you will incur bring the shirts to Poland and deliver them to the customer. Those costs will have to be passed on to the consumer, and probably exceed those of a mail-order business. There's a good reason why online stores are thriving and brick-and-mortar stores are failing.
Sorry, but the chances of success for your business idea are zero or worse. In Poland, clothing retail is a losing venture unless you are already established or are Vietnamese.
If your wife is telling you that you will be able to make a living in Poland as a foreigner that doesn't speak Polish very well, and apparently has no marketable skills that are in demand, she is deluded, and you would foolish to believe her. Poland does not treat fools kindly.
Stay in the UK, and visit Poland only for short vacations. Resist your wife's desires to move back to Poland even if it means separation or divorce. Unless you have truly marketable skills, then you have no business moving to Poland. And face it, someone who TYPES IN ALL CAPS is unlikely to have any marketable skills at all.