The pound did act that way to some extent, but only within the Sterling Area and only between 1931 to 1972.

Prior to that, people might have talked about the pound as if it were a reserve currency in the same way the USD is today but, as you say, actual foreign reserves were based on gold.

Very telling that even top economists are unable to imagine a world without a reserve currency when that is the state of nature (even in antiquity there were multiple currencies, not just gold)