Apple says that by default, Apple Pay uses the same technology as current contactless credit cards, so the process for the service will work the same as for the cards. An Apple Pay purchase will not require a Personal Identification Number (PIN) to be entered to complete the transaction.
Apple does note that the usual £20 limit on contactless transactions will apply, at least at first. This is the standard U.K. limit for contactless transactions. In September the limit increases to £30 for all contactless transactions, and this will include Apple Pay.
Transactions exceeding the £20/£30 limits will require either a PIN or a signature, unless the merchant’s reader supports a technology called “Consumer Device Cardholder Verification Method.” While this technology is being adopted slowly by merchants, as it requires new payment processors. When it is in place, the contactless payment from Apple Pay will go through, taking into account the added security of Touch ID, and therefore not needing a PIN or signature.