That's really unfortunate that they didn't sew it as those stitches on top would suggest.
But, I think if you get the right glue you should be able to fix it. I don't know what kind of glue you have tried already, but getting glue that works on fabric/leather can be tricky since it needs to bond fibers and smooth surfaces while also being flexible. However, I have repaired a bunch of different ear pads (like the Hifiman ones that are glued to plastic rings) with Loctite Fabric glue. Even though the glue area is small and it's under tension from the foam in the pads I repaired, it's worked great as long as I held it in place until the glue cured. (
https://www.loctiteproducts.com/en/...tite_vinyl_fabricplasticflexibleadhesive.html). However, I'm not 100% sure it's available in the UK. If that's the glue that you tried and it didn't work, that's about the extent of my repair ideas short of full reupholstering since any sort of sewing repair is going to be visible.