I think the distinction might come down to basting. If you definitely wouldn't baste the article, then you're baking it, if you do or might baste it, then you're roasting it.
Roast potatoes could easily be basted, baked potatoes definitely wouldn't be, basting bread seems absurd but basting a joint is obvious.
Regardless I think it should be obvious to all that a terminology that assigns the name "baked beef" to a joint done in a conventional oven is totally insane and nonsense.
After tossing peeled, sliced, sections of potato in some fat / oil before putting them in the oven, I wouldn't add further oil to them - does this mean they're not basted, or they are (by the original covering)?
Is beef wellington baked or roasted, by your definition?
You might not personally but you could keep basting them, it wouldn't turn them into a different dish or completely ruin them if you did. Compare with what would happen if you kept basting bread.
I've never cooked beef wellington so I don't really know, but it seems like it's baked and I imagine you're saying it's basted? As far as I can tell it's an egg glaze, which is not basting. Anyway, worst case scenario, my definition has some edge cases or needs nitpicking over the definition of a baste. This is nothing next to your insistence that a Sunday dinner includes baked beef.
I can't imagine basting a beef wellington, because of the pastry wrapper, but that doesn't mean you couldn't, so I was trying to work out what your definition came down to.
It sounds like you're saying that if you think something could be basted, then it's a roast, but if it couldn't, then it's baked.
If that's your intent, then it feels like the can's been kicked down the road to the comparably confusing definition of a 'bastable food', yes?
(To clarify -- this is because I'm assuming, per previous response, that it's not so much if something was actually basted, but whether it should / could be.)
Roast potatoes could easily be basted, baked potatoes definitely wouldn't be, basting bread seems absurd but basting a joint is obvious.
Regardless I think it should be obvious to all that a terminology that assigns the name "baked beef" to a joint done in a conventional oven is totally insane and nonsense.