I think white breeches would look much better than the brown ones. I also think it would look much better if either the jacket buttoned or I had some sort of tie. I don't want to look too manly, though.
The jacket I was wearing in that picture is really nice; it has tails and everything, but it's too small. I bought another one that looks a lot like this:
Except it's black. Do you think that would look better than the other one?
Before you read my answer know that I know nothing about formal driving apparel and what is proper. I am just saying what will look nice, since I am sure the bride has no idea what is proper either.
I like the tails but if that jacket does not fit right the new jacket will be wonderful. The white breeches will go nicely with either. The tan breeches are fine as a back up pair, the white will just look extra formal.
I can not tell from the photo what is going with your shirt at your neck. If you want to look more spiffy you could add a stock tie like dressage riders use. That would look nice.
If the rest of your outfit is black you should probably have a black belt too.