The government isn't going to change before the referendum.
The principle doesn't just apply to changes of government, it means that a commitment by parliament is only good so long as parliament consents to that commitment. It can be changed or overruled at any time.
Sure but for a Government being able to function depends on you being able to take their word to have at least some value. They have committed to this. I think it is reasonable to assume they will stick to it. Yes governments break promises, but I think this is one they will stick with.