Many years ago, in my long distant youth, it used to be M&S's
proud boast that all of their clothes were made in the U.K. In those days the quality was very good and the clothes where made in large reputable factories. We used to live near one and I had many a good dress for my daughter from the seconds there when we were young and quite broke as well as, of course, buying some of our clothes in their shops.
Of course, in those days, it was difficult to source where some of the materials came from, but as least by buying the finished product, people felt they were supporting British workers at a time, the 80s, when there was less work around.
However, as MawBe says. it would be far more difficult to know for sure what kind of industry you would be supporting now, even just considering the work entailed in producing the finished product.