Imagine Me & You; I certainly can't remember where I came across it, but I've only just watched it for the first time. What you know going in is pretty much represented in the movie poster: Piper Perabo and Lena Headey seem to be attracted to each other, but Piper is obviously getting married. So I was expecting a same-sex romantic comedy, which maybe it is, but it's also more serious than that.
The basic set-up is that Luce (Headey) is a florist in London delivering the flowers to a wedding, although she hasn't yet met the bridal couple because the bride's mother has made all those arrangements. Just as Rachel (Perabo) is walking down the aisle to marry the man who's been her boyfriend and best friend as long as she can remember, she and Luce see each other for the first time and are immediately drawn to each other. Rachel initially chalks it up to a simple connection with a kindred spirit, but before long she realizes it's more than that. Luce, who is openly gay, knows immediately what is happening but doesn't believe in mucking around with other people's relationships.
There's a lot that's both amusing and sweet in this movie, particularly Rachel's much younger sister, "H", who constantly asks questions about why people don't have dessert (or pudding) after breakfast, why the alphabet's in that particular order, and what happens when an irresistible force meets an unmovable object. But the movie is serious too. It would be a lot easier to root for Rachel and Luce if Heck, Rachel's husband, weren't the nicest guy on the planet (and pretty damn cute too). Rachel has no desire to hurt him, and certainly doesn't intend to leave him, but she also has the example of her parents' fairly loveless marriage right there in front of her.
Speaking of parents, I liked the relationship that Rachel has with her father, played by Anthony Head of Buffy the Vampire Slayer fame, and that Luce has with her mother, played by Sue Johnston. And even the standard shallow best man, Coop (Darren Boyd), is a pretty likeable shallow best man. The real highlights, though, are Boo Jackson as H (she hasn't been in any other film before or since), and the chemistry between Rachel and Luce.
On another note, I've found that one of the best ways to find little-known films that you're likely to like is to watch the previews on the DVDs of other little-known films. I'm pretty sure that's where my next movie review is coming from.