The exact same scenario actually happened late last year, in real life. A young man at Newark Liberty International Airport briefly crossed a security barrier to give his girlfriend a final goodbye kiss. Hell, it even happened at Christmas.
Of course, guards and fellow travelers, overwhelmed with such a display of true love and the Christmas spirit, surrounded the couple and clapped... Oh, wait, no. What really happened was that the breach was discovered, the terminal went into lockdown, and around 200 flights were delayed or canceled worldwide.
Once tracked down, the lovestruck man was given a fine and community service. A fair price to pay for romance, you say? Well, after this breach, everyone in New Jersey was so annoyed at this light punishment that moves to toughen laws are now advancing in the state legislature, which if passed could lead to up to 18 months imprisonment for similar acts in the future.
On top of all this, keep in mind that Love Actually takes place in the UK, where police have proved they are willing to shoot fleeing terrorist suspects in public, so it's not looking good for our plucky hero. And if that doesn't seem scary enough, unless you are blessed with the discerning power of Kanye West, you'd be hard pressed to find another crime that would earn you the immediate hatred of literally tens of thousands of people in your immediate surroundings.