Many EU country's have winter tyres mandated between various dates from autumn to spring. Many EU country's have minimum tread requirements for winter tyres if fitted (often 3 or 4mm). Basically a winter tyre ceases to be a winter tyre, if the tread is below a set limit. Most winter tyre manufacturers suggest a minimum of 4mm, below which most of the advantages over a summer tyre are lost. This leaves a glut of part worn, but UK legal tyres available all over Europe, which many do find there to the UK. The problem is, a legal but low tread winter tyre, is pointless as it will give less grip than a new summer or general purpose tyre.
So it's not worth wasting money on a potentially unsafe and low grip part worn, when quality new winter tyres are available at sensible prices.