I tend toward practical too!
For 'v posh', I think fabric quality first of all. If you're on a budget, then white is the hardest colour to pull off, the cheap synthetic has nowhere to hide.
White for a meal where you want to impress means care expended not spilling anything.
The skirt will ride up as you sit down, and when you stand up again that huge slit will be even higher than it already is. The skirt is tight, which leaves you visibly tugging it down.
A low cleavage will be easy to look down by any taller person seated close to you, and by everyone serving you. OP says small breasts mean more coverage, but is it enough more?
A bodycon dress limits opportunities for gluttony stomach expansion.
Exaggerated long sleeves will be a nuisance to stop trailing in the food.
All of this is before the fact that blatantly sexy will never be interesting or sophisticated, which is what OP said she wanted to convey.