I hate to rain on everyone's parade but the Marks and Spencers pictures currently posted in the last week are "default" sizes. If people want, I can upload (slightly) higher quality images in the original lossless format (other than JPG), that marksandspencers website relies on some clever altering of urls to get the max images.
However, Im still trying to figure out what resolution they originally shot the images in for the photoshoots, rather than assuming the server is handing me possibly upscaled images. it could be 2560px (default) or 3840px (theorizing)