I just did Thanksgiving for 8 people and my turkey of 6.2kg (13.6lb) was plenty. We even had a good amount of leftovers. I got it frozen from Tesco for £20 half-price because I couldn't afford a lovely one from the poulterer, but it turned out to be delicious anyway. Morrison's had frozen free range ones (that they were out of when I went

) for about £15-£20 as a half-price offer, but that was about 2-3 weeks ago.
My oven is one of those that is raised up and fits into the wall (we have a separate gas hob with drawers underneath rather than than oven) and it's not particularly large but I do think it is fairly standard. There is a separate grill/mini oven above it so the larger one may actually be smaller than your standard oven since it does not accommodate the grill. It
just fit the turkey in, although I did have a big foil "tent" over it a la Delia Smith, so that was partly why it took up the whole dang thing. Once the foil was off, there was a bit more space, but not really enough to cook anything else in there.
tl;dr shoot for 6kg and you should be fine on all fronts.
