In reality the domination of the SNP in Scotland is only a really serious disadvantage to Labour at Westminster as long as Labour declines to contemplate working even informally with other parties and sticks to the flawed idea that you must have a majority in parliament. The SNP members sent to Westminster are in essence an anti-Tory block of votes. The SNP have said on various occasions that they would vote with Labour on a case by case basis - and in most instances they would be very unlikely to vote with the Tory opposition against Labour.