I hated The Producers first time I saw it (typical self-righteous adolescent; I think it was the anti-German sentiment which offended me). It's the film about which my opinion has changed most. I'm not a great Mel Brooks fan, but there's something I can't help loving about that film. And nobody can deny it's quotable: "I'm wearing a cardboard belt!"
As for Lindsay Anderson, I don't _think_ he's an exclusively British phenomenon. I'm sure there are as many US lovers of his work than UK (but the population difference makes it so).
O Lucky Man is a bit of a problem film - I think it's probably a masterpiece but then it could be mediocre. Like Tarkovsky's Mirror, it's a film which rewards (or at least bears) repeated viewing.
If.... is essentially about revolution and is probably best appreciated by the repressed, British youths it depicts so accurately. I'm a bit stuck for things to say about it really, as I haven't seen it for a while. After seeing it I could talk about it for hours. What one person sees as dull and televisual another sees as gritty and realistic.
I won't try to convert people to Lindsay Anderson, but I certainly wouldn't hesitate to class him as one of the greatest directors.