Hogyan mondom udvariasan angolul levélben, hogy sok boldogságot kívánok, de sajnos nem tudok elmenni az esküvőre?

Please accept my warmest wishes for your wedding. Unfortunately, I must send my regrets, as I will be unable to attend what should be a magical day for both of you. Congratulations. (neved)

Az első mondatot én eszkábáltam össze, a többi részt egy külföldi illető javasolta.

Basically, it is well written but:

"My warmest wishes ON your wedding."

As a general rule, the preposition "on" is used when expressing one’s good wishes in the context of a happy event.

And although it's nothing wrong with "please accept my...." it reads, however, very formal. I would leave it out and just write:

"Warmest wishes on your wedding." Or

"Heartfelt congratulations on your wedding."