Just imagine it: a bridge across the Thames, from Tate Modern to the approach to St Paul's, covered in lush undulating grass taken from Althorp Park. Families picnic under its trees; a child's balloon bobbles in lucent air. Etched into the balustrades of the bridge are the words "Goodbye England's Rose", and the rest of the lyrics of Elton John's Candle in the Wind 98. What if the architectural practice known as FAT had been allowed to build their Diana Memorial Bridge here instead of Lord Foster's briefly wobbly high tech Blade of Light?
London is full of extraordinary buildings and structures. But it might have contained many more if finances, or nerve, hadn't collapsed – for example, Richard Rogers' steel and glass dance- -of-the-veils makeover for the South Bank will now never rise; nor will KPF's archictecturally steroidal scheme on the edge of Smithfield. We take the city's skyline for granted. We see St Paul's from the top of Fleet Street, framed by a visual cacophony of street signs. We barely glance out of the cab window as we sluice through Parliament Square. Would we notice more if some of London's most ghost buildings had survived the shredders of twitchy planners and febrile financiers?
The most daring of London's modern developer-visionaries, Lord Peter Palumbo, fought to get James Stirling's architecture at No 1 Poultry in the City built in the mid-Nineties – the first truly radical jolt of postmodern design in the Square Mile. But three decades earlier, his sponsorship of an even more startling building opposite Mansion House by the legendary Mies van der Rohe – the "less is more" glass and metal maestro – would have deposited a clone of New York's towering Seagram Building just metres away from a Palladian building described by the historian John Summerson as "a striking reminder that good taste was not a universal attribute in the 18th century."
The conservationist lobby, led by Marcus Binney, went into action. A telegram was sent to the Prince of Wales, then in Venice: "British architecture needs your help ... the Mies van der Rohe stump is likely to be approved ... another word from you, Sir, at this crucial time, could help save London." Palumbo's potential architectural spike was itself spiked.
Sometimes, London's most ambitious ghost buildings were impossible from the outset, even for the can-do Victorians. Philip Armstrong Tilden's we-beat-the-Bosch-so-let's-shop design for Selfridges in 1918, with its gigantic tower, was hilariously overblown. So, too, was John Pollard Seddon and Edward Beckitt Lamb's 1904 proposal for the Imperial Monumental Halls and Tower at Westminster, which would have towered over Parliament.
Joseph Paxton's supersized designs were more properly visionary, and no more so than the Great Victorian Way, a glazed mall – with shops, restaurants and elevated roads and railway – that would have encircled central London. Too big, too expensive, a before-its-time orbital Bluewater whose Grade I listed remains would have made the perfect mise-en-scene for dystopic novels by JG Ballard or Iain Sinclair.
Today, size still matters – and often in a fatal way, as far as radical building proposals in London are concerned. The American architect Rafael Vinoly's Battersea power station scheme featured an inhabited 300ft glass chimney rising out of a transparent canopy over a vast mixed-use development that would generate a hot air updraft with the same thrust as a Jumbo jet engine. But it seems that Boris Johnson has erased any possibility of a new chimney for Battersea, known in its smog-inducing heyday as "the flaming altar of a modern temple of power".
Brilliant strangeness can sink revolutionary schemes, too – the most outstanding example in the last decade being the so-called V&A Spiral, designed by Daniel Libeskind and Cecil Balmond. One commentator described it as "the Guggenheim in Bilbao turned on its side and then beaten senseless with a hammer". Its form and fractal-coated surface were simply too shock-of-the-new for the V&A and potential backers. Yet, its ghost persists, a monument to avant garde possibility.
Then, there is the up-yours ghost building scenario. And no architect can surpass the iconoclastic effrontery of Rem Koolhaas, whose polemical design entry for the Tate Modern competition was a masterfully ruthless critique of the very idea of an architectural homage to Sir Giles Gilbert Scott's Bankside power station. Koolhaas's scheme referred to "the British attitude towards the 'great' gesture, poised delicately between aversion and incompetence" and described Scott's building as an "abject" object lesson in symmetry, the only museum in the world marooned on a mud bank twice every 24 hours. He proposed "a Turner-esque blur of brick from which strategies of dissociation will have to rescue it".
The ghosts of London's "lost" buildings recall the walking dead flowing across London Bridge in TS Eliot's epic poem, The Waste Land. Architecture, undone by the machineries of process – the rows, the cobra-spit prejudices, the wheels-within-wheels that are, even now, conjuring up yet more architectural phantasms in London.