I'm not to familiar with UK marriage customs, but I imagine the following components have to be included:
- The holy person asks each of the two by name if they want to marry the other
- Both have to say yes/I do loud and clear
- Both have to sign a document
- The holy person has to declare both as wed
Anything I'm missing?
My solution would be to have a concert and during a pause have both of them invited back stage.
Give them make up and costumes so they aren't recognized.
Announce that we have a special supporting act for the next bit
Do a little song and dance number that includes a marriage ceremony (maybe a new interpretation of Romeo and Juliette's marriage with a fitting soundtrack)
Errect a few magic barriers before revealing this to be an actual marriage.
Getting the hell out of there

Profit
Regarding getting in contact:
In both cases I'd suggest the medium risk approach
If that doesn't work, I'd say we are throwing a party and invite them officially with lots of other people
