The £1.50 was set as a way of "incentivising" people to change to a DD. Using a card, debit or credit, has an additional admin time for our Membership Secretary. The actual cost to the Merchant Service provider is less.
I don't know what the proportion of international membership is but don't forget that no matter what the "incentive" direct debit is not available to people who do not have a UK bank account.
PS. We already cop the banks outrageous conversion fees without adding an "incentive".
