Hobbs and Quilter's Dream are among the best brands. Hobbs 80/20 is one of my favorites. When I Googled, it looks as if both these brands are available in the UK.
Quilter's Dream cotton (needlepunched, but not through scrim) is a very stable cotton batting that will not "beard". Hobbs 80/20 has more loft, whereas the QD gives a flatter, more traditional look. Both shrink up to 3%, which is what is typical for a traditional "crinkled" look. Neither of these battings requires pre-washing.
If your wife does not want the batting to shrink at all, and does not want to pre-wash batting, then she would need to stick with polyester. A disadvantage of polyester battings is that they slip and slide more in the quilt sandwich. I personally prefer 80 to 100% cotton in my battings, without scrim, because cotton gets softer and softer with every washing. If your wife wants a polyester batting, I've heard that both Hobbs polydown and Quilter's Dream poly get good ratings.