It looks like Amex do allow DCC as this was posted:

"From 21 March 2015, transactions charged and billed in pound Sterling at non-UK merchants will no longer attract 1 additional Membership Rewards point for every equivalent £1 spent. This includes transactions that are converted into pound Sterling by the merchant before being charged and billed (known as “dynamic currency conversion”). Please note that we will not remove any additional Membership Rewards points that have been awarded to you for such transactions up to and including 20 March 2015."