Why would parliament allow a Scottish referendum

Since the UK voted for Brexit, with Scotland voting overwhelmingly against but being outnumbered and forced to leave the EU because it was tethered to the much more populous England, common wisdom is that would make Scottish Independence likely. I have a friend in Scotland who thinks it’s all but inevitable.

But how exactly would it happen? Sure independence probably would win if there were another referendum. But everyone knows that, even people in the British government. So unless the members of parliament wanted to let Scotland go, why would they ever agree to a referendum?

I don’t get it. Even if we assume that most Scots want out, that’s not what it takes to make it happen. And what it would take to make it happen just doesn’t seem all that likely to me.