Complex sentence: These countries either used sterling as their currency, or else their own currency was pegged to the British pound; even member countries with their own currency held large sterling balances in London for the purposes of conducting overseas trade.
Split into simpler sentences: These countries either used sterling as their own local currency, or else their own local currency was at a peg to the British pound. And even in the cases where member countries used their own local currency, these countries would still hold large sterling balances in London for the purposes of conducting overseas trade.