I think there might still be complaints about the validity of a Declaration of Independence based on an election manifesto, because it would be argued that some people vote SNP regardless of that party's manifesto, who would have voted No in an Indyref. I don't see that as a valid objection, but I know that argument is advanced in opposition to the Election idea.
However, if a referendum was refused, the election manifesto might be the only recourse - always assuming a clear majority in favour of such a strategy. Did that happen in Ireland in 1918?
The subsequent events there are not propitious. Irish MPs constituted themselves as an Irish assembly in Dublin instead of going to Westminster; the assembly was declared illegal by the U.K. Government, and years of conflict ensued, ending in partition of the country and the establishment in most of it of a "Free State". But the Empire is now deceased, and I don't think the UK has the stomach for that sort of thing any more.