The 10-15% figure seems low, if you look at something like this:
View attachment 31026
This is for a shirt sold via a 3rd party (like Sports Direct). So in Coventry's case, we'd own the marketing, distribution & retail cut. So essentially everything but the manufacturing, VAT & cost numbers here.
If you total that up it's ~45%. So 60% probably closer to the truth than 10-15%. And note that the cost here likely is crudely measuring the raw cost of the materials - all those "departments" taking their cut have their own costs to pay. So gross profit much lower. Of course the mail would go with the headline grabbing "EIGHTY POUND SHIRT COSTS JUST EIGHT QUID!!!!!!" angle.