It seems clear both in the FAQs on the website and in the "more information" on the sales page that you either open "My Booking" and cancel it there or call the Customer Services number they show for your part of the world - in your case, the Rest of the World.
For your fare base:
Total flexibility on changes and cancellations
You can make all the changes you need to your booking, regarding both
the time and date, without having to pay any charges, via our website
or by calling our Customer Services. You can also change the name of
the ticket holder by calling Customer Services. In both cases, you
just have to pay the difference in the fare when you make the change
(if the fare available for the new flight is lower, no refund will be
made). You can also cancel your flight, providing you haven't already
completed one of the legs, and we'll refund the full fare (credit
card/PayPal fees and the purchase of any additional service not
included in the fare shall not be refunded).
I'm sure this isn't completely new to you. It seems clear to me that if the only two options are in the modify booking area or calling a particular department on a specific number then that's what they allow. I would be surprised if they responded favourably to emails or other means of communication.
