All of the supermarkets and health food shops have cheatin' versions of all of the classic side/veg dishes. Look in the frozen veg sections for standard items and also more festive options. There are also fresh Mediterranean, chargrilled veg and other ready-to-cook veg options... look in the fridges next to the vegetable aisles. Lots of the mainstream gravy and stuffing is vegan... check the ingredients lists.
Aunt Bessie Roast Potatoes
Stuffing (lots of the stuffings from the supermarkets are vegan... check the ingredients. For gluten-free check out the free-from sections and health food shops)
Free & Easy Gluten Free Gravy
Waitrose Cauliflower & Broccoli Bake
Waitrose Chargrilled Veg (most of the supermarkets have a Mediterranean or chargrilled veg option... look in the refrigerated section in the veg aisle)
Waitrose Vegan Mac n Greens