I don't recall seeing any Labour manifesto promise on how they'd vote on a war in the middle east.
When off manifesto, it is clear as day that parliamentarians should not be expected to tow any party line but instead vote ideally on critical evaluation of the situation bearing in mind canvassing of their constituents, or failing that, with their conscience.
Its a complete disconnect of representative democracy that the representatives, and by extension the electorate, are suppressed by the decisions of one or two.