The extract below is taken from the LGA's 2017 "A councillor’s workbook on planning" and provides definitive guidance on predetermination v predisposition.
There is nothing to stop a councillor having an opinion and expressing it - it would be bizarre and unreasonable not to - so long as they are open to new / alternative points of view. Don't be afraid to change your mind....
Bias, predisposition and predetermination
Pre-determination is where a councillor has a “closed mind” i.e. the mind is made up before having the opportunity to hear or consider all the relevant matters at committee. This could leave the committees decision susceptible to challenge by a judicial review. If a councillor has predetermined their position, they should withdraw from consideration or decision making for that matter. This would apply to any member who wanted to speak for or against a proposal as a campaigner or as an advocate speaking on behalf of constituents
Predisposition however reflects the common law position that a councillor may be predisposed – ie have an opinion - on a matter before it comes to committee, provided they remain open to listening to all the arguments presented at the meeting and the possibility of changing their mind as a result.
The Localism Act 2011 (S25) defined this further by indicating that a councillor should not be regarded as having a closed mind simply because they previously said or did something that directly or indirectly indicated the view that they might take in relation to a particular matter.