As I've said, Labour doesn't need to win an overall majority, or even win more seats than the tories to get into 10 Downing Street.
They just need to make it so Labour+SNP is a majority. Assuming the SNP keeps 40 seats or so, that means Labour needs roughly 280 seats or so. Still an uphill climb, but not a net pickup of 120+ seats and a 10 point lead nationally anymore
SNP would demand an independence referendum. Then, if they were to win it, Scotland would leave the UK and Labour would lose their majority mid-term.