Just guessing here, but I'd say there's a 90% chance you'd have no problem.

The immigration folks are the ones that scrutinize the passport closely, but they don't care about your bags. The customs folks are the ones that care about your bags and what's in them, but they don't scrutinize passports until after they've determined that there is a reason for concern.

Neither immigration nor customs really cares a whole lot about matching up alcohol with over-21 year olds.

Having said all of that, the 10% chance you'd get busted by a random search would be enough of a deterrent to not try it. You'd probably get hassled for a while and lose your booze. Theoretically, the local cops could also write you a ticket (MIP - Minor In Possession), but you'd at the least be out the hundred quid.

The OMNI question here is why someone under 21 is buying good champagne. It'd make more sense if you were trying to smuggle in a case of cheap beer.