Your OP sounds like you want a turkey dinner but are not a major cooker of roast dinners.
You can get turkey breast in most SM now. We got one 2 weeks ago for Thanksgiving, which was supposed to serve 4-6 but we got 1 sandwich for leftovers after 3 of us had dinner (only 2 adults). Honestly, if you got a breast, already stuffed, yes you will have some leftover but not loads (a few sambos, or maybe just enough to freeze a chunk to make a curry another time).
If you like ham with it, just get a small ham. Again, sandwiches, or something like a chicken and ham pie, or stilton and ham potato bake afterwards with any leftovers.
In terms of veg, yes the prepared trays may be too big. But what veg do you like? M&S have lots of lovely small packs of regular veg prepared (broccoli, mixed veg, sugar snap peas, French beans etc). You could get frozen mixes and only pour out enough for 2, keep the rest for another time. Just buy say 2 carrots and 1 parsnip to do roasted roots.
Roast potatoes are easy to do, most supermarkets will have some sort of pre-prepared version and usually have smaller batches as well as huge family ones, and there are the frozen versions too so only cook what you 2 will eat. Or get nice potatoes to steam, or make mash, or get dauphinoise or a gratin.
Buy ready made gravy, especially if you don't want a whole turkey so won't get giblets to make stock. Or buy a pouch of turkey stock to snazz up your regular Bisto or whatever. Pour in a slug of wine as you are cooking the gravy.
Or make whatever meal you 2 both really love - forget the turkey altogether and just get creative together in the kitchen!!