What’s the difference between saving in tally and saving in GBP?

Tally is a currency representing customer gold bullion ownership (1 tally = 1 milligram of gold). As the value of gold changes, so does the corresponding GBP value of your savings in a Tally Everyday Account. The quantity of gold owned does not change, only its GBP value.
When you save in GBP, you are saving in a debt-based currency. In the past six years alone, the value of GBP has approximately halved when compared with the value of gold.