Sure, pounds sterling had a different value in 1720 (although the question arises - a different value in comparison to what? The US dollar and the Euro didn't exist in 1720! You could take a set quantity of gold as your comparison, but the value of an oz of gold has fluctuated more than the value of a pound sterling over the same period).

However, it is all a bit immaterial. If the debt was denominated in pounds sterling, then it will be repaid in pounds sterling. If it was in guilders, then it will be repaid in guilders (or in sterling, at whatever the current nominal sterling/guilder exchange rate is).