Highway Code Paragraph 186
When taking any intermediate exit, unless signs or markings indicate otherwise
* select the appropriate lane on approach to and on the roundabout
* you should not normally need to signal on approach
* stay in this lane until you need to alter course to exit the roundabout
* signal left after you have passed the exit before the one you want.
So it's left hand lane to take the first exit.
The right hand lane to take an exit to the right, or go full circle.
The 'appropriate' lane to go straight over. Is that the left lane or the right lane?.
The 'appropriate' lane is down to your judgement and circumstances. Local knowledge and good observation really comes into play here.
My choice is usually down to the volume of traffic in each lane and their exit route. If the majority are turning left, I will take the right. If the majority are going left I will take the right. If it's a mix or majority straight on, I will pick the lane that is free-est flowing (usually the right lane, as most people assume left is the correct one) and pick my way through as safely as possible.
One of other thing of note, on a busy roundabout where there is slow moving/stop-start traffic, being in the right lane gives you a better opportunity to be the first out of the junction, especially if the gaps are too small for a car.