M&S is my favorite spot to hit when in town. Their food halls (located around the city) have individual-sized portions of many items prepacked. While I haven't seen GF sandwiches, I usually opt for an individual serving of chicken/turkey, some fruit, and chips/dessert from their "free from" section. Solo travelers who are celiac, this is a great stop to add to your list!
This place made visiting Paris so much easier for me! They have 4 different pre-made gluten-free sandwiches (so I too could have picnic lunches), plus a ton of other gluten-free food to purchase. Most of the takeaway Indian food is made without gluten, many of the prepared meats (e.g., Vietnamese pork belly, sausages, etc.) are gluten-free, and there are things like gluten-free Mac and cheese, steak pies, and quiche. Plus gluten-free scones, hot cross buns, muffins, crackers, and so on. I was very impressed. I miss it now that I’m gone.
Beware of the soft drinks and check labels before purchase. The M&S own brand diet cola for example contains gluten (which we found out after the bottle was opened).
Cafe section has jacket potatoes and salad options for a quick lunch. Food hall has a small selection of gluten free snacks and frozen meals.