What to do with £600 million – the National Fund case

In January 1928, an anonymous donor gifted £500,000 – the equivalent of around £33.5 million today, according to www.officialdata.org – on trust. The trustees were to hold the fund until it had grown sufficiently to pay off the national debt, which at the time stood at around £6.7 billion. By January 2022, the fund had grown to around £600 million, but the national debt had also grown to some £2,277 billion. How should the fund be used?