You might claim that this is nothing more than quantitative easing by another route.

That's fair. The comparison favours my thought-experiment. Under quantitative easing the Bank of England has pumped £375 billion (so far) into the financial system, inflating asset prices but otherwise without any clear idea what has been achieved, who has benefited and how we are going to exit from the process. Under my vouchers value would be pumped into the financial system, inflating asset prices, hiking the amount of saving in the economy, giving everyone the assurance of a reasonable retirement which was not dependent on the generosity of future politicians, partially recapitalising the banks, escaping the effects on the public finances of the coming demographic crisis and with a clearly defined exit via redemption. 

You might object that since the vouchers are a liability on the public finances, this idea represents a not-very-well camouflaged explosion in the National Debt.

Well, they are only debt in a Pickwickian sense. The vouchers do not actually add to the liabilities which the public finances have to support. Successive governments have had these obligations since 1946 when the current state pension was adopted. The only difference is that successive governments have not bothered to admit this. If a company treated its pension fund in the same way as the government, its directors would be guilty of false accounting. Through the vouchers an open-ended, unfunded and unquantified liability would be changed into something that is capped in real terms, hard and marketable and used to fund personal pensions for everyone. To that extent, the financial engineering involved could be called Qualitative Crystallisation.

From there, it becomes a numbers game.  This is how the transition would be managed.

First, pick a starting date, such as April 6, 2013. Next, pick a starting age, such as 20. Everyone who was that age on the starting date would receive vouchers with a value projected to accumulate over 50 years to a fund that could generate an income equal to the state pension for 30 years. (We might as well assume that everyone is going to live to be a centenarian—a nice margin for error.) Individuals, and their employers, could add further contributions, but even if nothing else was saved a decent retirement income would be available. In the following year, the next cohort of 20-year-olds would be brought into the new system.