I believe it is be more accurate to say that this is the swing the Conservatives would need across the country to make the seat they need for a majority winable. There is actually a huge difference between this and actually requiring this percentage across the board.
That they need this result has little to do with the boundary changes, though they will not help for sure, and is far more deep seated.
There are two major problems.
The first is that the Conservatives are in exactly the same position as the one Labour faced in the 80s and early 90s, in that they have been reduced to their strongholds. At that time Labour needed a substantial margin of victory for a 1 seat majority. As an asside it should be noted that this was not assisted by the then latest boundary changes which had increased an already present advantage for the Conservatives. (History, like sprouts, has a tendency to repeat.)
Yes, America is dealing with this in changing the rules of the Senate to let President Bush install judges on majority vote and bypass the "filibuster" rule, which permits a sizable minority to veto things. Then again, what goes around comes around.
The second is the rise of tactical voting, which collapsed the vote of the third party in many marginal constituancies and even some previously fairly safe Conservative seats in the 97 election simply to get rid of the Conservatives. As the general feeling in the country is that we aren't stupid (or forgetful) enough to have Howard, the non-blue vote is still dedicated to keeping the Conservatives out. This means that they pretty much one-on-one with the second favourite party in many 'must win' places, a problem Labour are less likely to encounter until there is a similar desire to remove them from power.
In these circumstances it is not surprising that they need over 40%, maybe it is surprising that it is not over 45% and a rod the Conservatives back very much of their own making.
So, your point is that the UK vote has a clear consensus of 60% anti-Conservative. Sounds about right.
The American system, which splits the executive branch from the legislative, deserves perhaps some respect. Knowing how to do the right thing is usually beyond the everyday concerns of mob rule, which is democracy in its purest and most nauseating form.
Although the Republicans control both the legislative and executive branches at this moment, many Americans are comfortable with split control, which is impossible in a parliamentary system.
The executive is expected to act, the legislature to frustrate. Or vice-versa, depending on the personality of the president. Not a bad system. It could accomodate Winston Churchill, Margaret Thatcher, or Tony Blair as head of state, while allowing Clement Atlee, Michael Foote, Karl Marx, or even Lib Dem Charles Kennedy as the legislative leader.
We Yanks ain't as dumb as we look sometimes.