Each time I spot a blue plaque – and whichever city I am in, my eye is inexorably drawn to those little markers of history, suspended between ground and first floors – I make an instant value judgement: is this a happy plaque or a sad plaque? I am not a natural dualist and am aware that every life contains multitudes, but I find this sort of crude categorisation irresistible, especially in London, which has been home to much fame, fortune and tragedy. A key indicator of a ‘sad’ plaque is the circumstances under which the person lived in that particular building. For instance, those plaques on the down-at-heel residences associated with the tumultuous early life of Dickens are sad plaques: memories rendered permanent in places he doubtless sought to forget.

Dickens has the second highest number of plaques across Britain – his forty-three pipped to the post by the forty-seven of Methodism’s peripatetic founder John Wesley. The city with the highest number is no surprise at all. While there are, of course, plenty of ‘happy’ plaques in the capital; London’s sad plaques are manifold – for it has always been a city where people have come to hide, fail or just forget.

The blue plaques tell a story belied by the leafiness of London’s more exclusive addresses, those places across the city’s centre and west which we associate with huge desirability. But what of those who didn’t wish to live there? How must Charles X, the last Bourbon king of France, have felt as he idled away the hours of exile in the comparative modesty of a Mayfair side street? What went through the minds of those heroes of the struggle against apartheid as they trudged towards the British Library bus stop in torrential rain? Did dreams of the Russian past – or even of a potential Russian future – flash across the consciousnesses of the myriad Grand Duchesses as they watched the honey drip slowly from the spoon and into the teacup on an endless Belgravian afternoon? London has been a city of hope and of aspiration from Whittington onwards – but those sad plaques remind us that even the most gilded parts of it have played the part of prison as well as paradise.

This isn’t only a historical phenomenon – London is still the nursemaid of exiles to this day. Constantine II, the last king of Greece potters around the lanes of Hampstead, while General Pervez Musharraf, erstwhile president of Pakistan, contents himself with a flat just off the Edgware Road. Strange to think of those minds which once contemplated the affairs of nations now having to preoccupy themselves with Oyster cards or the Old Street roundabout or any of the other strange minutiae of being a Londoner. Why London though? Well, perhaps it’s because of Britain’s unusual track record of political stability, or because you can hop on a direct flight to here, at speed, from almost anywhere, or maybe it’s because London really is an everywhere; a place where a rejected king or persecuted president might – as millions of their subjects have done  – make a corner of France or Pakistan or Russia down a side street or in a square.

Yet, for all the specific suitability of London’s bosom and for all her laudable ability to act as a canvas for diverse lives, these exilic plaques remain sad. ‘How can we sing the Lord’s song in a strange land?’– whether that casts your mind instantly to the Biblical Psalm or to Boney M, the essence of exile as a loss, as a necessary suppression of a part of who we are, has been an observable part of human culture since its very beginning. For all the gilt and greatness of Babylon (be it on the Euphrates or on the Thames), it remains an elsewhere, a strange and unchosen land.

Thomas Aquinas – a man who knew what it was to move, near constantly, from place to place –  wrote in his hymn to the Eucharistic presence, this prayer in anticipation of heaven: ‘Oh, grant us endless length of days, in our true native land with thee’. Maybe, for the moment, we’re all exiles after all.