If you used a gift card to pay for your tickets, only the remaining balance will be charged to your credit/debit card.
The payment confirmed in your confirmation email is the total cost of the tickets – the price displayed doesn’t knock off the value of the gift card, but please don’t worry, as the gift card will still have been applied.
If you’re not sure if your gift card payment was made correctly, just check the balance of the card! You can do that here.
You can also verify the amount charged to your credit/debit card by checking your online banking – the reference on your statement will be Ticketmaster Theatre London.