I say that you cannot beat a wrap to smarten up anything that you wear.
I have several that I wear with everything including trousers, even jeans.
They do not have to be expensive, they are far cheaper than a jacket, and can make even the most casual of outfits look chic. Maybe a friend has one that you can borrow?
Add a clutch bag ( Charity Shops are great places to find them, or again borrow one if you can ) and you are set to go, even wearing flat shoes which never go out of fashion.
I was always uncomfortable in these type of social occasions until I learned to stand tall and hold my head high. Believe me Jane, it really, really works as it exudes an air of confidence whatever you are wearing.
Have a wonderful time Jane ?.