I think a lot of people here are confusing the question 'Is this OK for a wedding?' with 'Would you, personally, choose this dress?'
Would I wear this to a wedding? No, it would look terrible on me and it's just not my style.
Is it suitable for a wedding? Yes, absolutely. Dress it up with some silver shoes and accessories and you're good to go. It's absolutely appropriate.
Mumsnet in general seems to have a very dated notion of what a wedding guest should wear. If it was down to Mumsnetters, nobody would attend weddings wearing anything but a jewel-toned dress from Coast, a pashmina, matchy-matchy shoes and a fascinator. In the real world, wedding outfits are a lot more varied and modern than that.
I went to a wedding in May and there was a huge range of different types of outfit going on among the guests. There were only two people whose outfits, to me, didn't feel 'appropriate' and they were on a teenage girl who was in a strappy body-con Lycra micro-mini dress, and a man who was inexplicably wearing walking trousers (the kind you get in Mountain Warehouse with zip up pockets) teamed with socks and sandals.