What is the purpose of these odd "control surfaces" on the BAE Taranis?
They look like spoilers but are more precisely called drag rudders. They are not meant to reduce lift like regular spoilers, but increase drag in order to create a yawing moment.

Regular aircraft have a vertical tail for yaw control. Without this, you need some force acting at a lateral distance from the center of gravity for yaw control, and this is what these speed brakes do. They are opened only on one side, and keeping both slightly open increases control power because then one can be closed while the other can be opened further.

The Northrop B-2 uses split ailerons to the same effect, and the Horten IX had a two-stage wooden speedbrake which could be extended near the wingtips. By raising only the smaller brake with small rudder pedal movements, yaw control could be tailored to flight speed. At high speed only small input was needed, and at low speed both brakes could be fully opened for maximum yaw control.



Horten IX V2 3-side view (picture source). Note the two extended speed brakes near the wingtip in the front view. The real speed brakes were on the lower side of the center wing and right behind the rear landing gear bay (in the drawing called spoilers).

Thanks to @RedGrittyBrick for the hint: Yes, the X-47A and X-47B also seem to have drag rudders. I guess their large chordwise dimension helps to limit the lift loss when they are extended.



Northrop-Grumman X-47B in flight (picture source)