Whistles use terrible fabric. Don't bother. I bought a silk/modal/viscose mix jumper there, but it was pilled and crumpled very quickly. I felt rather cheated, because it wasn't cheap.
Yes to Jaeger. Clothes I've had from there still look great.
Karen Millen shoes and bags are good, in my experience, but they use fewer natural fibres for the clothes these days. I will not be paying £95 for a sleeveless polyester georgette shirt, thanks. Why not silk, at that price?
The best quality clothes I own are, I'm sorry to say, the most expensive. MaxMara, Marella, Sportmax (anything from the MaxMara group, in other words) and Nicole Farhi. The fabrics are so good that the clothes hang better, crease less and last well.
You can get some gems from M&S if you hunt them out, though...takes a bit of rooting through all their confusing ranges but I've had beautiful wool crepe trousers from there.