The council is a separate legal entity in its own right, separate from the individuals who are elected to serve on it so decisions taken by the majority are binding on the minority. Using the title "councillor" on your social media profile implies to the public that you are speaking on behalf of the council so doing so risks potentially bringing the council into disrepute or at best misleading the public and could be considered a breach of the code of conduct (depending upon what yours actually says) so needs to be considered carefully. As a result, many councils have resolved (and even if they haven't) to ask councillors not to post as "councillor" anything other than an agreed statement. There is, however, nothing to stop you posting critical comment as an individual - that is absolutely your right to do so.
Small word of warning about pre-determination however. As councillors we are required to come to meetings with an open mind on anything that is to be discussed. If you have made a pretty forthright statement of opinion on any matter in advance of a decision it could be argued that you have a closed mind and shouldn't take part in any vote. It's a difficult one for anyone to prove but who needs the agro?