I think you've missed the second part of the OP's question "How should I style this and what should I wear over it if the weather cools?"
We don't know when the date is, we don't know where the OP lives, & if there's anything that can be predicted about UK weather, it's that it is mainly unpredictable ...
I'm in Edinburgh & we have had a few cooler evenings. If I was out for dinner, I'd still grab a lightweight jacket to throw on at midnight, especially if there's some walking/travelling involved.