Premium Plus Three
The clauses in the BA Amex T&C you are looking for are outlined in sections 3 and 7:
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.
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.
You will be asked to provide info on existing Amex accounts when you apply for the BA Amex.
Hope this helps.