I think it's easier to accept cultural norms when they support your own personal needs and values, without even knowing what they are or thinking about them. Looking at some of the changes to those norms in my lifetime:
Take being gay for example, as I was growing in the 60s and 70s, although homosexual acts were no longer illegal, it still was something society and straight people found "distasteful" and homophobia was probably seen as social norm. Because a large group of people were impacted by this very negative norm and challenged that status quo, and the arts and music world (those awful celebs,interestingly) took a more progressive view, that society norm shifted and changed.
Regional accents being seen as class indicator, or someone who was a bit thick. Again the impact of the media changed this perception at a both conscious and unconscious level. The driver behind this change was no doubt education being used as a mechanism to make society more equal, having a posh accent was no longer an indication of attainment of a good education. It was beneficial to the majority of people that this attitude or value judgement changed.
Cultural norms that are detrimental to a vocal or influential group of people will be challenged and changed. Sometimes a society recognises that a norm is unfair and disadvantages a minority group too and will work together to shift those norms.
It's interesting to see the number of discussions on MN around these such as SN or Feminism. Sometimes we take something we all think is not really a problem, but then see it from someone else's point of view and make an effort to change and adapt that norm.
There has been a lot of talk on this thread about some of the more negative norms in the UK - such as being overly polite, indirect in speech and non-confrontational. The more positive values that are a result of these norms are our ability to see something from someone else's point of view, embrace different cultures and values, and concern for the underdog.