Having a themed wedding forces guest to spend quite a lot of money on hiring, or buying, suitable attire to fit that theme.
In the case of Peaky Blinders, I doubt that there are many men's hire shops that could provide suitable clothing for all guys who're attending. That would mean that they'd each have to fork out hundreds of pounds for clothing that they might wear just once. I'd certainly hesitate to accept an invitation to such a wedding, as my finances simply aren't that good.
However I have to admit that it can look great when everyone goes along with the theme:
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