I post on a board with a high number of British posters, and one thing that's pointed out by a few is how if Scotland actually did become independent, the Labour electoral map toward victory becomes completely screwed because Labour are the nationally dominant party there (so far people haven't voted SNP en masse to Westminster) and the Conservatives don't exist in Scotland outside of the Borders. So Scottish independence means Labour would have to become more competitive in the South as far as their policies if they wanted to take power again.
The last time Labour won in 2005, they would've still won a majority government even without Scotland.
Yes, there is an anti-Labour shift if you remove Scotland, but it isn't that big. In both 2005 and 2010 Labour's lead over all other parties in terms of Scottish seats was 23; it might be a bit higher next time but it's not likely to be more than 40, so if Labour can do well enough to get a majority of 40 or so in the whole UK (i.e. a useful but not overwhelming majority) they'd still have done well enough to get a majority without Scotland.
Why is Labour's vote so efficient?
Labour's safe seats cluster around former industrial heartlands in the North, Wales and Greater London and areas with large minority and white-working class populations. The Tories don't have such a natural base. 1997-2010 also brought a new base of middle-class public sector workers and I'd imagine the collapse of the LibDems will firm this up, bringing the student vote back to the Labour electoral coalition as well.
The Tories have barely any presence in major cities like Liverpool, Manchester and Newcastle, but they tend to do better in safe Labour seats than Labour does in safe Tory seats. What good's 20% in Newcastle if it doesn't win you the seat? Many of the very safe Tory seats tend to have the Liberals in second with Labour languishing on 5-15% of the vote.
Also, safe Tory seats tend to get the highest turnout (+70%), while safe Labour seats tend to get the worst turnout. Manchester Central was the lowest last time, something like 45%. Then you have a safe Tory seat like Central Devon where turnout was 75%. Both candidates got about 52% of the vote: the Labour MP in Manchester only needed 21,000 votes, but the Tory MP in Devon needed nearly 28,000 votes.
As an aside as well, Labour's ground game tends to be much more effective than the Tories. They won quite a few seats last time that the bookies and the pollsters had written off for them.
