Vizeman said:
I guess they must invest your money until that time and then buy currency am I correct?
Almost. They will enter into a forward exchange contract (in simple terms, an agreement to buy a commodity - in this case currency - at a fixed price at a future date). They take your money today and invest it. They then buy the currency at the agreed (fixed, remember) rate at the future date. No risk, just profit for them.
As an individual, you could enter into a forward exchange contract in the same way and fix your price now,
but not pay until the future date. Of course, the rate you will get on your £500 holiday spending money will be nothing like that Crown can get on its bulk funds.
In either case (doing it through Crown, or doing it yourself), what you need to understand about a forward exchange contract is that what you are doing is eliminating the risk of the rate moving against you. What you can't do is guarantee it might not move in your favour. It's not about gambling, it's about certainty. In the context of holiday money it's insignificant, but if you are a businessman dealing in thousands, hundreds of thousands or even millions, it enables you to set your prices knowing that exchange rate movements can't wipe out your profits. If you contract to manufacture a product today at £X, you will not receive payment until you deliver your goods (or, most likely, 30, 60 or 90 days
after delivery). If you don't hedge against exchange rate movements, your profit could be obliterated.
It's a very simple premise that can be surprisingly difficult to grasp.