The weddings I have "witnessed"(Lecce, Rome, Perugia, Florence, Caserta, Matera ) - all in passing, were extremely well dressed ones. As someone else said, the women in the main party wore very high heels. The one in Lecce had the groom in a white suit and he had photos of him with almost all the beautiful young women... clearly it was his show!
I think it may be different in the villages and more rural small towns... I caught the end of a wedding in Stigliano once that was more low-key. Still well dressed but definitely more variation.
If it's an English wedding in Italy, most guests will be in your position anyway. If it's a genuine "Italian" wedding, I'd want to make sure I was as cool as possible but still very well dressed. No doubt you will be anyway!
How lovely to live there Nonnasusie.