hvered wrote: Loo/lu appears to be mud, cf. Lutetia, place of mud, the Roman name for Paris, so we might be looking at a high/low juxtaposition which is exactly what island denotes].
That explains Looe Island, another tidal causewayed island, west of Plymouth in Cornwall which could hardly look more man-made. Wiki and everyone else insists on looe meaning logh or somesuch, i.e. deep sea inlet, of which Cornwall has more than its fair share. But not here strangely. Cf. Fowey.

macausland wrote: Procopius called the mainland of Britain 'Brittia' as opposed to Brettania by which he may have meant Ireland.
Perhaps the ending '-ia' could be related to 'island' which could mean that 'Brittia' was the 'Bright' island which would tie in with other descriptions of it as 'Albion' supposedly taken from the white cliffs of the south east coast.
I was wondering about 'light' names because of Lugh who is supposed to be a multi-skilled Celtic trickster-god, probably a variation of Mercury/ Hermes. It would be much simpler to assume this so-called "Celtic god" is just another name for light, which is arguably the most important factor in human development. From a sailing point of view the coast of Portugal is a major challenge; such a vital sea-route would surely merit a string of lights and an army of beacon-tenders.
The Romans called Portugal Lusitania and Andalucia or Al-andalus, the mineral-rich south of Spain, is another 'Lus' word. The names may also be tin-related, tin being 'white lead' and used as mirrors as Jon et al. have noted. Britain is the land of Bright-Tin, being the island famous for cassiterite.