Because its not about disposable income its about pay. Fox kind of led people off course a bit with his disposable income comments, it will of course partly affect them but its not the aim to balance disposable income.
The cost of living has a much greater disparity than the % they are talking about affecting the change for. The standard of living for those employed is I would say better up north than down south, but those who think London is the be all and end all don't see what a horrible place to live it is for the majority. (My view of course others will disagree hence my comment disagreeing with those that think its great

)
Up north you tend to have cheaper and shorter commutes
You have lower cost housing, very considerably lower cost
Similar cost food etc but even that its a little cheaper typically
So why should you get exactly the same wage as someone who has to work on the edge of London with considerably higher outgoings? The private sector doesn't, it doesn't need to nor would it work if we had the same wages. I.E. if I could earn the same money commuting into London as working 10 miles from home why would I consider working in London? If the private sector worked like the public everyone and most businesses would be deserting London faster than rats up drain pipes, its bonkers expensive to employ and recruit staff in London, it cost a fortune per sq meter for office space for those staff. The staff face more disruption from services such as train breakdowns, the pollution is terrible for those that live and work in London etc
Skilled public sector workers will still be needed in the same numbers outside London, if they are paid 10% less do you think they will be worse off than those living or commuting into London. Hardly. As I pointed out previously the exact places they have had the issue recruiting public sector workers was IN London.