There is only a shortfall if all the liabilities have to be payed. They don't and never have done. The billion pound shortfall was and is nonsense. It makes so many assumptions its laughable. When NAPS was closed, the argument used to say it was unsustainable was based on all members having a life expectancy of 93 (women) and 92 (men). The actual life expectancy is a lot lower. Funny that.
ONS Life Expectancy UK