It's such a British rom-com

Because just about everyone is a recognizable face (well, to a Brit anyway) and they all have a good line or two that establishes them as a character. I'm not claiming this is true in all British films but you hardly see it at all in modern American films.
Anyway, in addition to liking the two leads so much and the most excellent Anthony Head ("If I'd killed her 30 years ago when we met I'd be out by now") - this is the reason I find the film so rewatchable.