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.
Tesco Plant Chef Sage & Onion Stuffing Balls
Sainsburys Plant Pioneers Potato Dauphinoise