I would have thought that, having decided to invest X amount of money per unit time, what matters for beating inflation is the interest you can get on it, not the size of X. Sixpence will fail as savings because it's 0.021% of your annual income, not because of inflation; even if you assumed the value of money was perfectly stable, it would take you a long time to build up any sort of reserve at that speed.

Inflation in England in this period was, as far as I know, remarkably low and <1%, even experiencing periods of apparent deflation. (Whether it beat Roman Egypt Sixpence compounding might go a decent way. See also Gregory Clark, Farewell to Alms:

However, in preindustrial England, and indeed in many preindustrial economies, inflation rates were low by modern standards. Figure 8.7 shows the English inflation rate from 1200 to 2000 over successive forty-year intervals. Before 1914 inflation rates rarely exceeded 2 percent per year, even in the period known as the Price Revolution, when the influx of silver from the New World helped drive up prices. In a country such as England, which had a highly regarded currency in the preindustrial era, the crown did not avail itself of the inflation tax, despite the close restrictions Parliament placed on its other tax revenues. Only in the twentieth century did significant inflation appear in England. By the late twentieth century annual inflation averaged 4–8 percent per year. Thus there has been a decline, not an improvement, in the quality of monetary management in England since the Industrial Revolution.

...Thus in Roman Egypt wheat prices roughly doubled between the beginning of the first century AD and the middle of the third.^12^ But that reflects an inflation rate of less than 0.3 percent per year.