Hello fly2hi,
I guess as you aren't in wales it would have to be the traditional big daffodil, for st davids day we did make a few narcissi buttonholes but they aren't the traditional flower everyone would look at and think "wales".
leave a card with your bride and groom on their wedding day telling them-
"llongyfarchiadau ar eich dydd priodas" which I promise you means congratulations on your wedding day!

Hwyl fawr! (bye)
Jo