It's just a temp hold that they do to make sure the payment method is credible, it'll come off the card in a couple of days (loads and LOADS of places do this)
The HMV faq's on their new website clearly state that orders placed by CARD are charged on shipping, only PAYPAL is charged when you order.
If you have been charged fully on your card, that has to be a mistake.
From the HMV new store site :
When will I be charged for my order?
If you are paying by credit or debit card, we won’t charge you until your order has been picked at our warehouse, at which point our acceptance of your order takes place. If you’ve ordered multiple items, we’ll send the in-stock items and charge for these as they are dispatched.
Please ensure that the expiry date on your payment card is after the anticipated dispatch date of your order as payment is taken upon picking your order. If the card has expired, payment will fail, and the order cancelled. If you need to update card details, please cancel the order and place a new order using the new card details, or
contact us if you require any assistance. Please provide a telephone number as we may need to contact you to process the changes.
If you are paying by PayPal you will be charged at the point of placing the order.