Italian Greyhound is right. It's not the leadership that you need to take issue with - the vast majority voted in favour of change - but a group within the House of Laity which represents a minority of actual Church members, but which was able to garner enough of the votes to swing the decision.
So how do you effect change? You get yourself elected onto your deanery synod. You become a church warden. You talk to your church wardens, priest, and anyone who'll listen and ensure that they know how you feel.
It's not really the same as not buying Nestle products - or, rather, the analogy only works to a point - because Nestle is a company which exists to make money. By withholding your custom you are reducing the company's profits and ultimately if enough people do that it will hopefully decide to change - but it won't change (or, rather, it's unlikely to do so) because it will suddenly develop some morals and do the right thing. If it changes, it'll be because people boycotting its products hit it where it hurts - in the bank. In the case of the church, however, by 'taking your custom elsewhere' you simply give a louder, stronger voice to those who argue against change... Of course, you do harm the church by taking away your contribution (whether that's financial, through time spent volunteering or whatever) but you don't make it more likely to change, in my view. If you compare it to an election, say, it's more like abstaining or spoiling your ballot than it is voting against, whereas the Nestle case is more like an active 'no' vote.
But of course I can only tell you how I look at things, and as Italian Greyhound says you must follow your conscience and do what you believe to be right. Have you tried talking to your priest about this? What does she advise?