You don't actually get charged £1 - it's just a "pre-auth" to check that the card is valid, and reduces your available credit by £1 for a few days, but doesn't actually charge the amount to your card account. After a few days, that £1 gets added back to your available credit. They then process a payment for the full amount at time of despatch. It's a common practice in the hotel sector, where they will take a pre-auth upon check-in (usually for the cost of accommodation, plus a bit to cover any potential extras), but don't actually charge your card account until you check-out (and they know what else you may have charged to your room and therefore the actual amount you owe them).
