A celebrity, Mary Berry, she seems pleasant and will have many anecdotes and we can discuss cakes and baking.
Historical character, Samuel Johnson, as all said that he was very good company.
Fictional character, I was going to say Inspector Montalbano, but he would only speak in Italian, and in any case does not like to chat when he eats dinner. That being the case I choose Father Brown he enjoys good food and can tell us about his mysterious murder cases.
I cannot say which GN member really, but Miss Adventure could liven the evening with a few Pam Ayres style poems I think.