National Savings and Investments is an executive agency of HM Treasury.
One can calculate from the published data how much money is actually invested with National Savings and Investments
http://www.nsandi.com/prize-checker
In July 2016 we had about £64m of prize money and about 2m actual prizes that made up the £64 prize pool and the odds of winning in the prize draw are 30,000 to 1.(0.00003333333333333330)
Total Number of Bonds = Total Number of Prizes / Odds for the draw
Thus:
Total Number of Bonds = 2,000,000 / 0.00003333333333333330
60,000,000,000 are the total number of bonds.
Now each Bond is worth £1.
Thus we been able to calculate that total value invested with National Savings and Investments is £60 Billion.