Sadly I don't think there's a solution for the darkness you see in films, when you watch via the DVD. I can watch the same film on DVD (and it's almost unviewable, as it's so dark), then watch the film on BBC HD or Sky HD and it's brighter, sharper and far easier to watch. Which implies it's an issue with the DVD player and/or how it decodes and displays the movie content.
Spoke to my dealer about it and their answer was a simple 'the film was designed like that and BBC/Sky simply up the brightness' blah blah. Not true as if you watch the same DVD through an Playstation 3, the quality is closer to that of BBC/Sky than that of the BV7 DVD player, implying it's definitely an issue with the player.
According to a couple of posts here, the BV7-40 MKIII doesn't suffer from this problem and the DVD playback is supposed to be superb.