Lemon and elderflower? That’s an... umm interesting choice.
I’m a pastry chef and, there’s a reason why it’s not a common flavor choice for cakes or desserts in general. Those are two very strong flavors who can easily overpower the other.
I’d understand just lemon. Well I hope the cake is elderflower while the filling is lemon curd, that might work. Still strange flavor choice.
Also buttercream? I guess. I’ve Made many wedding cakes, and people of harry’s Monterey bracket do not use buttercream on their cake, no self respecting pastry chef would dare use buttercream for such an expensive wedding- it’s considered tacky, and is the cheaper option vs. the more luxurious fondant. It is easier and faster to work with though- hence why buttercream are cheaper than fondant ones, the decorations on a fondant cake alone can take days.
Also, when it comes to wedding cakes it is best to stick to traditional flavors (especially with large weddings- anything over 100 ppl), that’s why there’s been a boom at how the cake looks vs. flavors as a way to showcase the couple personalities.