In fact, it is through the assumptions that the MPC members take ownership of the forecast, by making the final decision about the assumptions. To briefly describe the process: the modelling team, in conjunction with the various units in the Bank, will analyse the relevant data, and bring proposals to a meeting with the MPC members.
Each assumption is then discussed and the reasons interrogated, and where deemed appropriate, the MPC members may require a change in the assumptions. Of course it is possible that MPC members may have differing views. Under such circumstances the majority view will prevail, but such differences will ultimately be reflected in the balance of risks to the forecast.