Aside from the counter arguments that English politics would inevitably re-centre itself rather than sustain one party rule, that there may be greater turnout by Labour supporters to end such one party rule, or that it's not Scotland's place to push England leftwards, it is simply factually incorrect that Westminster would not produce Labour majorities without Scotland.
The following table shows what the results would be for each general election from 1945 to 2010 had Scotland not been taking part in those elections. There is no accounting for the effects of the previously made arguments, this is simply what the overall results would have been if you were to remove the Scottish seats from the final result each time.
|1945||UK||393||210||12||25||Lab maj 146|
|- Scotland||356||183||12||18||Lab maj 143|
|1950||UK||315||298||9||3||Lab maj 5|
|- Scotland||278||267||7||2||Lab maj 2|
|1951||UK||295||321||6||3||Con maj 17|
|- Scotland||260||286||5||3||Con maj 18|
|1955||UK||277||345||6||2||Con maj 60|
|- Scotland||243||309||5||2||Con maj 59|
|1959||UK||258||365||6||1||Con maj 100|
|- Scotland||220||334||5||0||Con maj 109|
|1964||UK||317||304||9||0||Lab maj 4|
|- Scotland||274||280||5||0||Con maj 1|
|1966||UK||364||253||12||1||Lab maj 98|
|- Scotland||318||233||7||1||Lab maj 77|
|1970||UK||288||330||6||6||Con maj 30|
|- Scotland||244||307||3||5||Con maj 55|
|Feb 1974||UK||301||297||14||23||Lab short 33|
|- Scotland||261||276||11||16||Con short 12|
|Oct 1974||UK||319||277||13||26||Lab maj 3|
|- Scotland||278||261||10||15||Lab short 8|
|1979||UK||269||339||11||16||Con maj 43|
|- Scotland||225||317||8||14||Con maj 70|
|1983||UK||209||397||23||21||Con maj 144|
|- Scotland||168||376||15||19||Con maj 174|
|1987||UK||229||376||22||23||Con maj 102|
|- Scotland||179||366||13||20||Con maj 154|
|1992||UK||271||336||20||24||Con maj 21|
|- Scotland||222||325||11||21||Con maj 71|
|1997||UK||418||165||46||30||Lab maj 177|
|- Scotland||362||165||36||24||Lab maj 137|
|2001||UK||413||166||52||28||Lab maj 167|
|- Scotland||358||165||42||22||Lab maj 129|
|2005||UK||355||198||62||31||Lab maj 64|
|- Scotland||314||197||51||25||Lab maj 41|
|2010||UK||258||306||57||29||Con short 38|
|- Scotland||217||305||46||23||Con maj 19|
As can be seen, there are only two occasions out of the eighteen general elections held since 1945 where the exclusion of Scotland would result in a different party winning the most seats - 1964 and February 1974. These were both highly marginal results as they were, 1964 producing a tiny Labour majority and February 1974 producing a hung parliament that resulted in a second general election eight months later.
As well as those two, the only other significantly different results would be October 1974 where Labour is now slightly short of a majority instead of having a tiny one, and the most recent election in 2010 where the Conservatives win a small majority instead of the hung parliament situation that actually occured.
That leaves fourteen of the last eighteen elections where the overall result would have been the same. Perhaps there would have been smaller Labour majorities, but the likes of the 1997, 2001 and even 2005 general elections all would have still produced strong Labour governments.
From this, it is clear that the Tories would not be destined to "rule forever" in the remaining UK if Scotland were to become independent in 2014.