Nice Christmas *and* anniversary present!
1. Celebrity allows two bottles of wine per stateroom to be brought aboard at embarkation, so your champagne is fine. However, be aware that if you want to drink it in a restaurant a corkage fee may be imposed.
2. You don't have to take excursions anywhere if you do not want to do so.
3. Gatwick is a long haul from Southampton. Look here
for some advice on transfers.
4. I personally don't think the drink packages are worth it. Cocktails are $6 to $10, wine about the same, and beer runs from $3.50 to $6.