There are no legal routes to independence. We cannot challenge the British government over its refusal to grant a Section 30 because there is no mechanism in law with which to do this, and there is no constitutional requirement of the British government to do it either. This was always a fiction. Scotland does not have the same relationship of consent with the London government the people of the north of Ireland have in the Good Friday Agreement. Britain does not legally require the consent of the Scottish people to government Scotland.
Theresa May is many things but she is no Tudor autocrat. She is in the middle of proposing far-reaching changes to the constitutional settlement of the UK that will effectively relegate Scotland, Wales, and Northern Ireland to regions of the state.
Scotland and Northern Ireland can only be pulled so far before their governments will be forced to protect the interests of their people, and from their decision to remain within the European Union at the Brexit referendum these governments have a mandate to resist Westminster.