There were some dark murmurings on Channel 4 News last night that Theresa May has revealed herself in private to be a Thatcher-style hardline zealot militant Brit Nat fundamentalist who might even be reluctant to "grant" a second independence referendum if the Scottish Parliament votes for it. During her regal visit today, she went no further than trotting out the tired "you'll have had your referendum" line, but it might still be an apt moment to point out to her why it's not actually possible for London to "block" a referendum, whatever she might have been led to believe...
1) Some (but not all) expert legal opinion states that the Scottish Parliament already has the power to hold a consultative referendum on independence, if the legislation is framed with great care. At the very least, there would be a fighting chance of overcoming any legal hurdles and getting a referendum held without any involvement from the UK government at all.
2) If that doesn't work, there is the option of an early Holyrood election, which could double as a de facto independence referendum if the pro-independence parties state in their manifestos that they are seeking an outright mandate for independence. The First Minister doesn't have the power to literally call an early election, but given the current parliamentary arithmetic it would be easy enough to bring one about, as long as the Greens were on board with the strategy. It would simply be a case of the Scottish Government resigning, and the SNP and Greens between them preventing an alternative government from being formed.
3) If the UK government refuses to accept a mandate for independence from either a consultative referendum or an early Holyrood election, there is then the nuclear option of the SNP withdrawing its MPs from Westminster, which would leave Scotland almost totally unrepresented in the UK parliament. Even the headbanger tendency in the London commentariat would come to acknowledge that the United Kingdom is not viable if one "partner" in the union has effectively withdrawn from the country's institutions. Perhaps there might be one last appeal to the Scottish people to turf the abstentionist SNP out at the next UK general election, but if that call went unheeded, the game would essentially be up.
Now, I'm not saying that any of the above is remotely desirable or likely. But the fact that the SNP have all these weapons in their armoury is a good reason for thinking that Theresa May is less likely to attempt to "block" a referendum than perhaps she even realises herself yet.