Obviously, the real-time exchange rate in new york market is higher than that in London market, that is, the pound is higher in new york market and lower in London market. Arbitrators can use dollars to buy pounds in London market (at the bid price) and then sell pounds in new york market (at the bid price) to reduce costs, which is arbitrage profit:
1721000/1.7210 *1.7311721000 =/.