There must have been a very strong reason for not installing lights here as it is well-documented that lights are safer than roundabouts due to drivers relying on human judgement at roundabouts and not a definite command as at traffic lights. Safety is the main priority here, (as all junctions) so will have been considered initially.
This roundabout has cost more possibly due to the purchase of the land ?
Normally lights are more expensive, due to the on going electrical costs/maintenance of controls and where there is pedestrian traffic at a four arm structure, crossings have to be installed at a cost of £40,000 each x 4 ! This does not apply to Britannia obviously .. not many pedestrians up there ...
Still as we all say, better than nowt.