David, with regards to Natalie's jerk of an ex-boyfriend:
David: You know, as Prime Minister, I could just have him murdered.
David: Ruthless, trained killers are just a phonecall away.
Like a lot that happens in Daniel and Sam's plot, this one is bittersweet almost to the point of Mood Whiplash. Daniel and Karen are discussing Sam and how he always hides in his room, obviously crying because his mother has passed away, and Daniel not knowing just how he should talk to him. Then at one point we get,
Daniel: He could be injecting heroin into his eyeballs.
Karen: At the age of eleven?
Daniel: All right, maybe not his eyeballs. Maybe just his veins.
"My horrid son Bernard stays in his room all the time. Thank goodness!"
"Worse than the total agony of being in love?!"
The discussion of Nativity play roles:
Daisy: We've been given our parts in the nativity play, and I'm the lobster!
Karen: The lobster?
Karen: In the nativity play?
Daisy: Yeah, first lobster.
Karen: There was more than one lobster present at the birth of Jesus?
And when we actually see the play, it turns out to include three lobsters, an octopus, a whale, penguins, and Spider-Man.
Practically anything Billy says or does. Just one of many is:
Billy: Kids, here's a message from your Uncle Bill: don't buy drugs. Become a pop star and they give you them for free.
Another great one, from his radio interview:
Host: Best shag you ever had?
Billy: Britney Spears.
Billy: Just kidding. She was rubbish.
In his sequence with Ant and Dec, there's another casual one-liner:
Billy: Thank you Ant or Dec.
Billy writing a giant speech bubble on the Blue poster reading "WE'VE GOT LITTLE PRICKS".
Joe: Ten minutes at Elton John's, you're as gay as a maypole!
Billy's rendition of Wet Wet Wet's "Love Is All Around" with Christmas references shoehorned in so he can release it as a Christmas Song. On a meta level as well, as the song appears on the soundtrack album.
David dancing to "Jump (For My Love)" by the Pointer Sisters.
The Prime Minister ringing doorbells to try and find Natalie.
Mia: You're not who I think you are - are you?
PM: Yes, I'm afraid I am - and I'm sorry about all the cockups. Not my fault, my cabinet are absolute crap. We hope to do better next year.
Plus his on-the-spot rendition of "Good King Wenceslas" for some little girls who want a Christmas carol; his bodyguard then joins in with a robust voice that surprises even David.
Jaime and Aurelia's dialogues in the cottage where they say the exact same things in their respective languages, unaware of how in tune they are. The funniest moment is when they talk about his book and Aurelia suggests in Portuguese he give her 50% of the profits for her part in saving the first draft from a lake, and he immediately follows that with a suggestion to give her 5% of the profits in English.
When Karen catches Harry in the jewellery aisle...
"Don't worry, my expectations are not that high after 20 years of Mr 'Oh but you've always loved scarves'."
And halfway audible under the music, "Actually, I do like that one."
Jamie arriving at his family's house on Christmas Eve, only to suddenly announce he is leaving, to the outrage of his nieces and nephews, who start shouting, "I hate Uncle Jamie!"
The fact that Colin's insane plan actually works out for him.
The Prime Minister answering the phone after delivering his big speech:
"Yes, I'm very busy and important, how can I help you?"