Well your "double majority" point is an interesting one - It would depend on whether or not the Independence party that intends to offer a Parlimentary free vote on Independence clearly stated that in their election manifesto, so that on winning the double majority - the vote could proceed. ----- maybe !
Scotland has a "Devolved Parliment" governed by the Scotland Act 1998 and within that act is Schedule 5 Reserved matters
http://www.opsi.gov.uk/acts/acts1998...046_en_14#sch5
Basically the UK parliament is sovereign and it is for that Parliament to decide whether or not to hold a referendum on any particular issue.
The UK has by and large has an unwritten constitution (it is actually a rag bag of many different documents, precedences and procedures defined over our history)
It would take a large majority and an active Independence movement within Scotland for the UK Parliment to grant a referendum on the issue.
It would be a messy divorce ......
"O flower of Scotland
When will we see
Your like again
That fought and died for
Your wee bit hill and glen
And stood against him
Proud Edward's army
And sent him homeward
Tae think again ....."