Happily, the two best croissants I've had are here in south London. The Cake Boy's in somewhat inaccessible Battersea Reach are pretty good. But now I've discovered Des Lys in Balham. I ate mine heated briefly in the oven the day after buying it. It had the perfect mix of crumbly crisp exterior and stretchy, buttery, yeasty interior. The quiche was pretty good too, though the macaron was a bit heavy.
The patisserie is run by a very nice Parisian couple.