Islands always intrigue, hovering on the horizons of our imaginations – seen, according to your lights, as territories to be taken, ancient redoubts, repositories of secrets, even loci of lands of youth. Where there are no islands, we often imagine them – Plato’s Atlantis, the Celts’ Avalon, the Irish Hy-Brasil, Zeno’s Friseland, Columbus’s Antillia – and occasionally find them, like Terra Australis Incognita, postulated long before Europeans made landfall.

Britain was once itself an imagined island – or rather islands plurally, called by Pliny Britanniae, one archipelago among others in the great geographer’s speculative world atlas. Alice Albinia, the author of 2012’s highly regarded Empires of the Indus, now comes home to find all these Britannias for herself – the islands off this island nation, the offshore outcrops of this offshore outcrop.

She tells of islands elsewhere, but here her ports of call are Orkney, Anglesey, Wight, Iona, Thanet, Shetland, Lindisfarne, the Hebrides, Rathlin, Scilly, Man and the Channel Islands – and Thorney, where Westminster’s fanes first rose above brambles and reeds between Thames and Tyburn, and whose culture arguably still partakes of oozy mire.

Some of these islands once mattered more than the larger landmass to which they are now often seen as mere appendages. Orkney was a trading destination long before London was thought of; Anglesey a Druidic dreamland; Iona the epicentre of Celtic Christianity; Lindisfarne an intellectual and artistic powerhouse; Shetland a haven of relatively liberal Udal law. They still possess a weather-defying power of their own – similar to the Manx and Channel Islanders who stubbornly hold on to controversial fiscal freedoms and local rights.

Islanders, Albinia insists, are outward-looking by necessity. But she decries some for at least partly defining themselves by their geographical situation – those who take pride in long presence in one place, or Brexit voters and asylum sceptics quixotically seeking to defend Shakespeare’s ‘fortress’ against ‘infection’. Thanet was an island until the River Wantsum silted up in the 17th century, the historic landing place for military and moral colonisers, from Caesar via St Augustine and Viking raiders to today’s small boats. The author clops showily on to the ex-island in a horse and cart, part of a travelling circus, to reprehend anti-Romany prejudice and, less predictably, pay overdue tribute to the overlooked women who founded churches connecting Anglo-Saxon England to Europe.

Islands can be magnets for outcasts or, equally, conservative refuges, like Scilly, garrisoned for the Stuarts throughout the civil wars. Others can be prisons, like St Kilda, to which Lady Grange was exiled in 1732 by her adulterous husband (whom she had threatened to expose as a Jacobite), and Skye, where she died 13 lonely years later. Jersey was used by Cromwell to lock up Levellers, and later by Hitler, clinging obsessively to the only parts of Britain he could claim, incidentally turning Alderney into a death camp.

Like all explorers, the author carries cargo and personal preoccupations – searching for suppressed stories among the Christian and patriarchal sea wrack of mainstream history. She makes memorable connections, meets some engaging people and offers some salutary observations, although she emphasises apartness and diversity to such a degree that for her there might be no ‘mainland’ anywhere.

She is fascinated by Albina, the goddess supposed to have given her name to Albion – emblematic to her of femaleness forgotten, ‘feminist utopias’ swept aside by misogynistic early Christians, centripetal cultures and the ‘suffocating prevalence of male egos’. Romans minted coins showing Britannia as warrior queen – arguably a chivalric allusion to a gallant and sometimes female-led foe, but to this author really just a memory of rapine.

She raises tantalising traditions of female augury and cultic worship erased by (male) missionaries, although persisting as folk traditions, place names and sexually explicit Sheela-na-gig carvings, or metamorphosing into pilgrim wells, such as the one devoted to St Dwynwen, from which she sips warily in Anglesey. Prehistoric tombs might really be symbolic wombs, and the V-shaped carvings called ‘arrowheads’ by (male) archaeologists remind her instead of vulvas. She gravitates towards biodynamic believers, ‘witchy healer women’, lake-ladies, mermaids and even drag artists, despising all distinctions on modern metropolitan principle. Sometimes she flows so feyly feminine that Britain might be bobbing in amniotic fluid rather than more bracing brine.

She drops anchor eventually at Thorney (Westminster), where she was born – home too to the (only nominatively non-sexist) ‘Mother of Parliaments’, guarded by Thomas Thornycroft’s bronze Boudica. The overbuilt island at the centre of an island nation, or nation of islands, elicits authorial outrage – the ‘internationally condemned’ Illegal Migration Bill, the ‘silly, redundant and repressive’ House of Lords, the King as ‘a toxic fusion of patriarchy and heredity’. Happily, assistance is at hand, thanks to Extinction Rebellion, ‘women from four different continents’ offering ‘their menstrual blood to the river’, and Chilean artist-poets working in rope. It feels an anticlimactic arrival after a voyage so frequently worthwhile.