SUPERB!
Oh, well done you! Isn't it a wonderful feeling? A lot of that 7lb will be water weight, I guess, but that you've successfully shifted so much in Week One indicates that you've got a pretty good metabolism, and should be able to keep on dropping weight pretty regularly - Yay!
Well, cake would be an unfortunate choice, because it's inevitably going to be laden with flour and sugar - champagne would, I think, actually be the lesser of the two evils from that point of view. If you're lucky, they'll have strawberries and cream, in which case eat all you want, or perhaps they'll have a cheese platter, or something like that? You MAY be able to get through this without going off-plan, depending on what you eat beforehand and on what kind of food pressures you're under.
Definitely eat beforehand. I wouldn't say you need to make yourself feel sick - but I WOULD say make sure you're pleasantly full. You can have a slap up full English breakfast, you know, complete with coffee and cream, and still be on plan. Treat yourself to something lovely for breakfast - scrambled eggs and smoked salmon, or whatever kind of thing you'd really enjoy on a special occasion. And you could take along some almonds or something to snack on, in case of emergencies - I mean, sure they're not ideal induction foods, but they beat wedding cake!
That said - I think that the thing is, even though you're still doing induction, there are events at which one sometimes decides to make some compromises. Maybe you'll get through the wedding without being tempted - I've successfully been to various food-centric events and managed to stay on-plan, but sometimes I HAVE gone slightly off-plan. But it's been in a controlled way - my parents came out to visit me in Bangkok (for the first time) whilst I was doing induction, and there were several times when I simply didn't have control over what the food choices were, so I had to make the best of it.
In my 3rd week of Atkins, one of the kids in my class had a birthday and brought in a MAGNIFICENT chocolate birthday cake. Which I cut up into slices for everyone, and there was enough leftover for a piece for my TA and one for me. And I was tempted - I mean, it was a damn fine looking cake. But I asked myself, out loud, which I'd rather have - the cake right now, or lose a pound that week. And when I looked at it like that, it suddenly was easy. I didn't even lick any of the chocolate off my fingers - I didn't WANT it. I wanted to be unfat much more than I wanted that fleeting sweetness on my tongue.
Good luck!