There was already an Iraq affect in the 2005 election; but it looks as if that went more to the Lib Dems. Compare the poll with the 2005 results:
Party Seats Gain Loss Net Votes % +/-%
Labour 41 0 5 -5 922,402 39.5 -4.5
Lib Dem 11 2 0 +2 528,076 22.6 +6.3
SNP 6 2 0 +2 412,267 17.7 -2.4
Con 1 1 0 +1 369,388 15.8 +0.2
http://news.bbc.co.uk/1/shared/vote2005/html/region_7.stmNote that the Lib Dems were the only large party to significantly increase their vote in 2005, compared to 2001; the figure in this poll (Lab 35%, SNP 26%, Con 19%, LD 14%) mainly show a big increase for the SNP and loss for the LDs; I'd put that down the the SNP running Holyrood relatively compentently, and probably the fading of Iraq in voters' minds. But the main point is that in Westminster elections, Labour has always outpolled the SNP. This poll has them set to do their best ever in a UK-wide election.