Hi OP if you haven’t been to a wedding as an adult, as you imply, or even a formal wedding (register office pub being informal but to that I would even dress smartly) you need to wear a classy dress like you could also wear to the races. Some of the races you have to be covered.
so if you want to blend in just buy something from Hobbs or phase eight.
what is your daughter wearing do you need advice for her too?