I'll be honest op especially as you say a formal wedding and your oh is best man I think that's just too casual
I used to work in the industry.
Is it a church wedding? If so many churches would prefer visitors not wear quite such a revealing top either, especially for a formal occasion.
I mean I doubt anyone would say anything to you on the day out of politeness, but I think people would think it was too casual
And yes black is considered a faux pas for many Uk weddings
Doesn't have to be a dress.
Can be a smart top with a smart skirt or even trousers
Somewhere between office wear and pub casual is where you want to hit the mark. Hard to make suggestions without knowing your details though.