We are currently 6 months or so from the election in May 2015.
Sourced from
http://ukpollingreport.co.uk/voting-intention-2005-2010
6 months before election in May 2010:----------------(C%------L%-----LD%):
YouGov/Telegraph--2009-11-26----------------------------39-------29--------19
Opinium/--2009-11-20-----------------------------------------38-------30--------12
Ipsos-MORI/Observer--2009-11-15-----------------------37------31--------17
ICM/Guardian--2009-11-15----------------------------------42-------29--------19
Opinium/--2009-11-13------------------------------------------39-------29-------16
YouGov/Sunday Times--2009-11-13---------------------41-------27-------18
ComRes/Independent on Sunday--2009-11-12-------39-------25-------17
Populus/Times--2009-11-08---------------------------------39-------29-------18
As you can see, Tory's were always well ahead of Labour, and ended up winning with 36% of the vote.
Labour ended up with 29%, roughly what they were polling 6 months out (as you can see above). In this case, the party in power was not polling low. Lib Dems got 23%. Tory's polled a bit high, Lib Dems a bit low, Labour pretty much on the money.
In the 2005 election, the share of the vote was Con-32% Lab-35% LD-22%
sourced from
http://ukpollingreport.co.uk/historical-polls/voting-intention-2001-2005
6 months before the 2005 election, the polls were (C%---L%---LD%):
YouGov/Telegraph--25/11/04-------------------------------32 35 23
CommRes/Independent on Sunday--25/11/04-------31 42 20
ICM/Guardian--14/11/04--------------------------------------30 38 22
NOP/Independent-14/11/04---------------------------------30 39 20
MORI--08/11/04-------------------------------------------------31 35 23
Populus/Times--07/11/04------------------------------------33 34 22
As you can see, half of the polls were spot on, the other half had
the government of the day polling high. The Tory poll nubers were either on the money, or within 1-2 points.
So that's the 2 previous elections, where the incumbent government did not poll high and the Tory's did not poll low.