Congrats philley - hope you have a super trip.
With these things, it's obviously a hugely special occasion for you, but if you think about the size of BA, they are carrying multiple brides & grooms to be every day of the year, and to multuple destinations within their route network. The same goes for honeymooners - they are probably fairly consistent on many routes, particularly destinations such as CUN.
So I think my felow FT'ers are right - dont expect anything special or free, and if you wangle it on the day then take it as a bonus.
What you might like to consider is writing a trip report on your flight out there, and reporting back on FT. At least your fiancee can't threaten divorce if you sneak it in quick on the outbound flight... ^