If you are fond of shopping in Marks & Spencers, which is a large department store chain based in UK with branches all over the world, then you will find eight of them in the areas where hotels in Athens can generally be found. If you need a last-minute item that you just got to buy in the store, then you can definitely find one near you when you are out to explore the wonderful world of the Acropolis, the Agora, the Amphiaraion or the Saronic Islands. If you are not sure, your Athens hotel will be glad to help.
Although you may find that most of the Marks & Spencer stores in Athens don't have many English-speaking personnnel, if you will just be adventurous enough in finding the items you need by yourself, you will be okay. You can even make shopping one of the many activities in Athens that you could enjoy right inside Marks & Spencer, by simply taking in their Greek-speaking staff as a pleasant local experience. This way, you won't have to dwell on the negative side of their being unable to address you in English and you won't have to go back to your Athens accommodations fuming.