I agree with golfmad that this seems a little strange. For example, in the USA there's currently a promotion whereby if you use PayPal in conjunction with your Amex Platinum card then you get up to a $30 per month statement credit. Obviously that's there and the OP's issue is Amex UK but I would doubt there's any blanket prohibition on using your Amex card when it comes to using it in conjunction with PayPal.