PIC: Jennifer Aniston, Justin Theroux Hold Hands in London

Credit: Splash News

Have boyfriend, will travel!

Flying to London to promote Horrible Bosses, Jennifer Aniston didn't leave home without Justin Theroux, her hot-and-heavy beau of a few months.

Clad in a sexy Valentino gown Tuesday, Aniston, 42, braved the red carpet solo. After changing into a new, black frock, she attended a private dinner afterwards at members-only club Shoreditch House, hand-in-hand with Theroux, 37.

"Jennifer was really chilled out and seemed blissfully happy," a witness at Shoreditch House tells Us Weekly. She and Theroux walked through the rooftop bar -- where she grabbed a martini -- and was "grinning at everyone who turned to look at her," the observer adds.

Aniston's Horrible Bosses costar Charlie Day -- a dental hygienist tormented and sexually harassed by Aniston's character -- told reporters that Theroux didn't mind seeing Aniston and Day get into sexy situations in the raunchy comedy.

"I met him for the first time today," Day said on the red carpet in London. "He was very polite...he's a big boy, he can handle it!"