I'm not quite sure if I'd call Brown dishonest. I mean, he isn't exactly a pillar of integrity, but I'm not sure I'd say he's a liar. He can be undoubtedly one-eyed (pun intended) and also sometimes views things through rose-tinted spectacles (e.g. his constant touting of unemployment decline over the past 10 years as a Labour triumph). But I'm not sure I'd quite call him a liar. Compare Blair for instance who consciously told something he knew wasn't true to Parliament.
Cameron's a lightweight. He always has been. But on the other hand I don't think there's anything too bad about politicians seeing that they've been proved wrong. So if the Tories change their stance on regulation in light of recent events, I don't see that it's quite the same as lying so much as just pragmatism. It's very difficult to be right from the beginning. Sometimes events you can't see coming prove you wrong (and then everyone calls you a liar...)
Of course, they haven't acknowledged in any way that they made a mistake, which is extremely annoying.
Bear in mind it's very easy to call a politician a liar - but put yourself in the positon of a politician on the campaign trail. Imagine that you have a really great policy idea that you firmly believe in, then when you get into office, you suddenly discover there's some impracticality that means you can't do it, or some other important event springs up before it etc. I'm not sure politicians are always actively telling lies while campaigning (though naturally the 'moral majority' will dismiss any kind of optimism toward people is always dismissed as 'naive'...)