Blue with...??

White shirt. Navy suit.
Don't match her dress.....it's a wedding..not a prom!!

Maybe the tie, but since you don't know the shade yet, it is hard to say. A navy tie would look good.

 

Don't do the pocket square though, at least not a solid. Solid squares are typically reserved for solid white/cream (linen or silk), and sometimes a solid silk maroon or something like that. If you don't know what you are doing with pocket squares, I would stick with white linen, or cream silk for evening.

This is easy, bro. White shirty, navy suit. You don't need to match her dress. Just wear a white pocket square. It's not prom, it's not cosplay, it's not an art project...

Complimenting her dress doesn't have to mean matching it. If you really want to coordinate with what she is wearing maybe pick a complimenting color that she isn't necessarily wearing and roll with it. I dunno, just a thought.

I like the idea of you wearing grey w/ her blue, don't even try to match. However, maybe your tie could match her scarf or something along those lines.

Medium / Light grey suit, pale blue shirt... Maybe tweed or herringbone grey, navy blue or black tie etc.  Maybe some browns / creams in there.

 

Browsed for inspiration:

 

I would strongly suggest that you avoid neckties that match a bridesmaid's dress. I don't know if this is true in the UK, but dark suits with ties matched to the bridesmaid's dresses is a pretty frequent look for the groomsmen. If you're not in the wedding party, I wouldn't do it. The etiquette may be totally different for you, of course.

Wear a small flower in your lapel buttonhole that matches / complements her dress or eyes.

Darker brown, yes. Tan, no.

I would avoid tan brogues, especially if the wedding is at night. Unadorned black shoes are generally better for evening wear. You can do dark brown also, but try to avoid full brogues.