I would say that you have a number of choices. Initially you sould buy FairTrade greens through the UK based organisations that deal with farmers and smallholders in coffee growing countries. Through certification you can be sure that the growers are being treated farily.
Secondly you could donate a porportion of the profit from those forementioned coffees to organisations providing a direct bridge to those in origin countries. That may be through big NGO's or church groups.
Finally, and maybe further down the track, you can follow the example of roasters based in buying countries, follow the examples of companies such as Deans Beans - that is visit and develop a real and longterm relationship with growers (in Deans case in Aceh). Relationship coffee= knowing your growers intimately and ultimatley working with them directly, paying them fair and ethical prices for their coffee, is the future of specialty coffee globally.