For all the fixation on his comment about American Libertarianism, Banks's politics seem fairly standard post-Thatcher Scottish to me. After what that PM did to Scottish communities many Scots apparently feel they have little in common with the direction English conservatives are taking the UK.
Banks was a supporter of Independence and famously cut up his British Passport and sent it to Downing Street in protest of the Iraq invasion.
IMO you can see his politics all over the Culture; from the entire tragedy of Look To Windward to the wee things.