I fancy Carol singing too. (midnight service on Christmas Eve is never enough!) Would go for the Wassailing as well (only know one Wassailing song though ) as long as it includes copious amounts of libation (for the vocal chords you understand) ie more of a Wassailing pub crawl.
Unladylike Ms. Jo is the one to get on board for this, bet she knows all the tunes and she can play the tin whistle too.
I'm up for wassailing. I wouldn't say I specifically knew any tunes, but I expect I have some music at home, and if not I know where to get some: the Barbican Library has an excellent collection of sheet music, or there's Cecil Sharpe House library for traditional English stuff.
I may even be able to persuade some folky fiddly guitary friends to participate in such a suitably mad caper (we'll leave out the bagpipes this time, I think).