This domain is more complex that this Hinrikus guy makes it out to be. You can not be expected to be able to trade at the mid-market price, and the market for a volatile currency pair such as GBP/EUR easily moves several hundred times per second at times. Additionally, the rates the bank gets quoted depends on which exchange they are talking to, and the deal the bank has with them. The assertion that "there’s only one interbank rate at any point in time" is flat out false. Exactly which price does Hinrikus want his bank to quote him?

In fact, the bank offers a service where it takes small positions such as 1000 GBP and gives EUR in return. This is the deal. This small transaction does not necessarily change the bank's plans for foreign exchange.

How does the bank set the price ("rate")? To some competitive value where they expect to make a profit or not lose too much, depending on the competition. The assertion that they do not profit from it might very well be true, even though it probably shouldn't be.

Compare, if you will, to gas stations. The current price for petrol is dependent on what it currently costs on the world wide markets, but also dependent on the demand in the specific area, for example. It is complex, is all I'm saying :)