Much simpler than that.
'Way back' before the goldrush. The Bank of England was truly a reserve bank and fixed the price of gold. A gold bar was worth £1m

(Sorry no traders )

I dont know if these bars physically exist anymore but one was put on display during the millennium celebrations. Id say it was about 2 foot by 1ft x 6 inches high. Probably the equivalent of about 20 regular 'ingots' (which I happen to know are 9 inches x 3 inches x 2 inches).

And that reminds me of one more old term - 'brick'.
Have a guess at where that came from? - Ive already given a clue.