My point is that polls before an election or referendum almost always underestimate support for the status quo. Particularly with general elections polling very often favours the opposition. Labour need the polls to be massively more in their favour going into the election IMO
I suppose it depends how far out you are from the election. In 2010, the election was on the 6th May. Final vote share (%) was Con 36.1 Lab 29 LD 23 with a Con Lead of 7. So let's look at the YouGov polls from the end of March 2010 to the end of April 2010. (I'm ignoring the ones in the final few days before the election) -- I chose the end of March because that's where we are now.
date C L LD con-lead
YouGov/Sun 2010-03-29 39 32 18 7
YouGov/Sun 2010-03-30 38 31 19 7
YouGov/Sun 2010-03-31 38 32 19 6
YouGov/Sun 2010-04-01 39 31 19 8
YouGov/STim 2010-04-02 39 29 20 10
YouGov/Sun 2010-04-05 41 31 18 10
YouGov/Sun 2010-04-06 40 32 17 8
YouGov/Sun 2010-04-07 37 32 19 5
YouGov/Sun 2010-04-08 40 31 18 9
YouGov/Sun 2010-04-09 40 30 20 10
YouGov/STim 2010-04-10 40 32 18 8
YouGov/Sun 2010-04-11 37 31 20 6
YouGov/Sun 2010-04-12 39 33 20 6
YouGov/Sun 2010-04-13 39 31 20 8
YouGov/Sun 2010-04-14 41 32 18 9
YouGov/Sun 2010-04-15 37 31 22 6
YouGov/Sun 2010-04-16 33 28 30 3
YouGov/STim 2010-04-17 33 30 29 3
YouGov/Sun 2010-04-18 32 26 33 -1
YouGov/Sun 2010-04-19 33 27 31 2
YouGov/Sun 2010-04-20 31 26 34 -3
YouGov/Sun 2010-04-21 33 27 31 2
YouGov/Sun 2010-04-22 34 29 28 5
YouGov/Sun 2010-04-23 34 29 29 5
YouGov/STim 2010-04-24 35 27 28 7
YouGov/Sun 2010-04-25 34 28 30 4
YouGov/Sun 2010-04-26 33 28 29 4
YouGov/Sun 2010-04-27 33 29 28 4
YouGov/Sun 2010-04-28 34 27 31 3
YouGov/Sun 2010-04-29 34 27 28 6
YouGov/Sun 2010-04-30 34 28 28 6
I count 31 polls there. The 'status quo' of that time was a Lab government. In the first 16 of those polls, the status quo was not under-estimated (i.e. Lab polled at or above their final vote share of 29%). As the election got a bit nearer, from mid April to the end of April, Lab were under-estimated in 11 of 15 polls (but only under-estimated by 1-3 points, which I would say is within the margin of error for poll samples).
So the polls from around the end of March through to the end of April didn't really under-estimate the vote share for the government of the day.
So let's move to the opposition of the time, the Cons. Remember, they ended up on 36% as a share of the vote. The first 16 of those 31 polls did over-estimate support for the opposition. 6 of those polls over-estimated by 4 points or more, and I would say that is outside the margin of error and a genuine over-estimate of support. Again, things changed mid-April, with the last 15 polls of that period actually under-estimating support for the main opposition (though most of the under-estimates were within the margin of error).
I would take 2 things from this data and the point you made. Firstly, the Cons don't want to rely on the incumbent being under-estimated in the polls this close to the election. It didn't really happen last time out. Secondly, Lab will hope that they are polling around 34% in a couple of weeks time. If the last election is anything to go by, then their support will comfortably hold if this is the case.
Remember, it's the Cons that need to be leading by a few percentage points in terms of vote share if they want to be forming the next government -- and given the polling in marginals, even then it'd be tight for them. Lab have the swing in England (from Con to Lab) and the SNP in Scotland willing to freeze out the Tories.
Therefore, Labour do not need to be further ahead in the polls at this time (though they would like to be, obviously, as then they could get a majority).
Right now, things are still neck and neck -- the Tories need the polls to start moving in their direction and stay that way.