So long as you report your card stolen or lost, you will not be liable and as soon as it is reported, Barclaycard would block its continued use.
From the Barclaycard site...
safe and secure payments
Just like Chip & PIN payments, you'll enjoy the same fraud protection on contactless payments. So if your card is misused by someone without your permission, you won't be liable for losses*. To be extra careful, we'll occasionally ask you to enter your PIN to complete a payment.
<p class="disclaimer">* You will be liable if you and any additional cardholder negligently give or provide access to the card, account or PIN details which results in those details being used fraudulently.