A constitutional law question - the union

If the UK government refuses to allow Scotland and northern northern ireland to hold referendums, what other legal options might be open to unpicking the union?

Maybe Ireland could take it up in an international law setting, although I can't see that happening.

What about the Scottish parliament revoking the act of union based on a Scottish vote? Why not?

"Subject to paragraph 3, the Secretary of State shall exercise the power under paragraph 1 if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland."

 

Interesting thing is the appears likely to him.  It seems to be a shall do something if X power with a degree of discretion in the X.  But not unlimited.  So if there are now more nationalist than unionist MPs and there was a unionist vote proper of 41% it is something the SoS should at least be having a think about.