I have...
Size 12 Hobbs black dress, probably around mid-calf length, diamante straps and diamante trim around neckline. Somewhere I also have black sandals with diamante trim that go very well with this.
Size 12 Monsoon lilac dress (a large-ish 12 as it's Monsoon), satin, halter-neck, full length.
Size 12 Karen Millen silver dress, probably around mid-calf length. I have never worn this as even when I was a size 12 I suspected it was very unforgiving to any bulges unless you had seriously engineered underwear on top.
Size 12 bottle-green proper ball-dress sort of thing with a relatively full skirt.
Size 14 I mostly have some fairly plain Monsoon silk longish (mid-calf to ankle length) dresses in blue or green. Not sure they are glam enough for your purposes though. And some more knee-length things but again I suspect not glam enough.
If you went up to a 16 I have a lovely black and white Monsoon dress from this year (I didn't think it was as big as Monsoon 16s tend to be, so it might fit someone smaller. Or I may have just been deluding myself). Also I have a black velvet halter-neck with a long split in a 16 and a navy velvet with diamante straps and a drapy neckline in a 16.
I shall continue hunting (in my searches I have discovered a maternity evening thing that I'd forgotten I had, so it's not been an unproductive day...)