The answer is in the terms and conditions (on Amex's website):
CARDS
If you apply for a Premium Card we reserve the right to send you a Credit Card instead if we determine that it is not for any reason appropriate to issue you with a Premium Card.
If you apply for a Premium Plus Card we reserve the right to send you a Credit or Premium Card (at our discretion) if we determine that it is for any reason not appropriate to issue you with a Premium Plus Card.
The Card will be issued in the version applicable to you as set out below:
Credit One: A Credit Card issued to a Cardmember who is not an existing AMEX customer or an existing AMEX customer who holds only UK Corporate charge cards.
Credit Two: A Credit Card issued to a Cardmember who holds any AMEX UK charge card or UK credit account card, other than a UK Corporate card.
Premium One: A Premium Card issued to a Cardmember who is not an existing AMEX customer or an existing AMEX customer who holds only UK Corporate charge cards.
Premium Two: A Premium Card issued to a Cardmember who holds an AMEX UK charge card or UK credit account card, other than a UK Gold charge card, a UK Platinum charge card or UK Centurion card, excluding UK Corporate cards.
Premium Three: A Premium Card issued to a Cardmember who holds an AMEX UK Gold charge card, a UK Platinum charge card or UK Centurion card, excluding UK Corporate cards.
Premium Plus One: A Premium Plus Card issued to a Cardmember who is not an existing AMEX customer or an existing AMEX customer who holds only UK Corporate charge cards.
Premium Plus Two: A Premium Plus Card issued to a Cardmember who holds an AMEX UK charge card or UK credit account card, other than a UK Platinum charge card or UK Centurion card, excluding UK Corporate cards.
Premium Plus Three: A Premium Plus Card issued to a Cardmember who holds an Amex UK Platinum charge card or UK Centurion card, excluding UK Corporate cards.
FEES
Credit Card
For the Credit One Card no fee will be payable for the first 12 months from the date we open your account. Thereafter a fee of £20 per year will be payable by you, subject to 7.1 (c) below.
For the Credit Two Card, no fee will be payable at any time providing that the required Companion Card is maintained. If the required Companion Card is not maintained, a £20 fee will become payable in respect of the Credit Two Card during the second and subsequent 12 months, subject to 7.1 (c) below.
For the Credit One and Credit Two Card, no fee will be payable in any given year providing the total amount of transactions (including Balance Transfers) in the previous year (measured from the Card anniversary date ) was £4,000 or more.
Premium Card
For the Premium One Card a fee of £60 per year will be payable by you.
For the Premium Two Card, a fee of £20 per year will be payable by you, providing that the required Companion Card is maintained. If the required Companion Card is not maintained, a £60 fee will become payable in respect of the Premium Two Card during the second and subsequent 12 months.
For the Premium Three Card, no fee will be payable at any time providing that the required Companion card is maintained. If the required Companion card is not maintained a £60 fee will become payable in respect of the Premium Three Card during the second and subsequent 12 months.
Premium Plus Card
For the Premium Plus One Card a fee of £120 per year will be payable by you.
For the Premium Plus Two Card, a fee of £30 per year will be payable by you, providing that the required Companion Card is maintained. If the required Companion Card is not maintained, a £120 fee will become payable in respect of the Premium Plus Two Card during the second and subsequent 12 months.
For the Premium Plus Three Card, no fee will be payable at any time providing that the required Companion Card is maintained. If the required Companion Card is not maintained a £120 fee will become payable in respect of the Premium Plus Three Card during the second and subsequent 12 months.