I would suggest not 'SNP" per se but a party that was espousing many of the non-Scottish-specific policies of the SNP. Progressive liberal, I think they'd be in broad terms using current political jargon. Basically, we didn't have a party pushing that line in 'the UK', we only had it in Scotland. Anyone in the rest of the UK who wanted to vote for a party along those lines rather than the current orthodoxy of more or less strident conservatism were left with indies or The Greens, and may not actually even have had those choices to vote for.
There's some discussion to be had whether the SNP's landslide in Scotland was only down to the growth in nationalism in Scotland, or was in some part down to the fact they were offering an actual alternative to the Tories/Labour/UKIP, who were all effectively offering slightly varying degrees of the same set of policies.
It's a question that simply hasn't been looked at in any detail - 'How many English people would have voted for a party
like the SNP given a chance, and why?' The Labour party certainly don't seem to want to answer it, their panic reaction to the election was 'we have to be more like the party who won!', totally ignoring the point that being 'like the tories' is never going to get you anywhere if people also have the Tories as an option to vote for.
Jon