It was transwikied from- http://lifeafterpeople.wikia.com/wiki/London-_The_post_human_flooding.
OK, this is not cannon to the TV show or film, but it would make an interesting addition and is factually sound.
Without due care the London would floods would get worse over time. Whilst some places like the low-lying Isle of Grain, Canvey Island, Hoo peninsular, Benfleet and Isle of Sheppey would flood more frequently, others further up stream like Plummeted, the Greenwich peninsular, West Ham and the Isle of Dogs would once more become permanent swamps and tidal marshlands.
The phenomena at play
Some possible factors that could come in to play-
Most of the Thames was surrounded by at leads a few feet of flood plain or marshland. Large tracts were swamps or mud flats were running all the way from the River Lea’s outflow to Canvey Island and the Isle of Grain.
Others existed along the river Roding in Redbridge and at the River Lea as far as Enfield Lock. These included Plumstead Mars, Rediff Marsh, Westham Abby marsh, the Plaistow level and Rainham marsh. Lambeth was a moorland, with a costal swamp (Lambeth Marsh) and inland swamp (Lower Marsh) in the 1300’s.
The old Isle of Dogs, Stepney Marsh, Blackwall Level, Bugsby's Marsh and North Greenwich Marsh were drained in the 17th century and joined up as the then farming peninsulas that are now the Isle of Dogs and the Greenwich peninsula. The marshes were mostly reclaimed by the 1880's and totaly drained by World War 2, but would revert if the dykes, drains and embankments failed. Thames Embankment (at Westminster) was overwhelmed and part of the Chelsea Embankment did collapse in the flood of 1923.
Teddington, Chiswick, Putney, Hammersmith, Westmister, Southwark, Barns, Battersea and Richmond (in Surrey) flood on occasion and some incidents have occurred as far north as Oxford, but any permanent mass flooding by the Thames would not get past about Battersea.
Other moorlands and bogs
Moorefield was a bog from which the Wallbrook takes most of its water from. The River Brent has a habit of flooding in Hendon, Perivale, Brentford and Hanwell. Parts of Hampstead heath were marshland and the River Wandle used to flood Wandle Park, Carshalton Pond and Waddon Marsh when they were farmland.
The Thames Barrier, Chatham Sea Wall and Canvey Island Sea Wall.
The Thames [Flood] Barrier, Chatham Sea Wall, Canvey Island Sea Walls- 1880's (fell apart by the early 1960s and demolished), 1947 (demolished in the 1950s), 1954, 1970's and 1980's walls also do ther bit, but with out human help...
The post human floods
A possible long turn flood scenario would run like this-
1 second after man-
It all begins!
1 days after man
Electrisaty suppies fails, so pumps and fans fail.
1 to 5 Days after man
London Underground quickly floods as the pumps fail or are destroyed. It fills with rain water, aquifer water and water excess from the Thames as well as sewerage from broken sewers. Sections are flooded as the rivers Fleet, Wall Brook, Bayswater, Lambourne, Kilburn, New River, Beverly, Tyburn, Ranelagh and Westborne all burst out of damaged pipes to in the next few days. London Underground's tunnels fully flood out with in a few years.
Much of the coastal land between the Isle of Canvey, the Isle of Grain, the Grenwich Peninsular and the Isle of Dogs, like Plumstead Marsh, steadily becomes wetter as dykes and pumps fail, leading to it becoming a swamp in a few more years.
Food starts to rot. Mice, rats, cocroaches, pidgions, crows, dogs, cats and flies feast on it.
6-9 month after man
Caravans and porta-cabins rot and collapse.
1 year after man
Food finally rots off. The mainly prefabricated plaster board, light wight and wood built buildings also rot. Bigger brick and concrete buildings hold on the former signs and vehicles are now rusty. The window of the offices falls down, as do the commercial neon Street signs. Rotting wooden bill boards collapses as well.
2 years after man
Rotting sewerage makes the city's basements very stinky and slippery.
3 years after man
Most paint has drained out and flaked off, swimming pools are now swamps and rooms are covered in a thick layer of leaves and dust. Parks and soccer fields are now fields. Vehicles rot down to there tiers, engine block, windscreens, plastic bits and under-frame.
4 year after man
The River Lee Navigation canals, the New River flood elevation canal and the related storm drainage channels finally block up with debris from up stream causing it to flood in to its former marshland. The Canvy island and Chatham flood-wall collapse due to lack of care.
5 years after man
All vehicels have rusted down to only there glass and plastic parts.
6 Years after man
Heavy rain and neglect causes trouble and floods in Catford, Lewisham and parts of Woolworth. Costal Kent and Essex (particularly Isle of Canvey, the Isle of Grain, Benfleet and Thurrock) are flooded out and revert to marshland. The local sewers fail the streets are awash with long forgotten rotting sewage.
7 years after man
Remaining sewers, water mains, culverted rivers and piped rivers break out due to a lack of maintenance.
The Serpentine becomes polluted and reverts to nature. Highgate and Hampstead Ponds are less polluted, but still radioactive and revert to nature. Tokyngton Park in Tokyngton is flooded and also returns to nature
10 years after man
The Thames overwhelms any rubble or remaining dykes and lock gates. Sewage and drainage outflow pipes become blocked or tidally inverted at high tides.
The Thames flood barrier begins to decay badly. The first major building begins to collapse. Office windows break, leaving them vulnerable to plant life and animals. Greenery has covered London in shrubbery.
11 years after man
The sewers in the tidal zones fail due to poor maintenance, so the streets and rivers are awash with the remaining long forgotten sewage. The already damaged Brent Resiviour's dam-wall collapses and it returns to nature..
12 years after man
Most of the River Brent floods as it reclaims it's flood plane.
15 years after man
The Newham LB an Redbridge LB parts of River Roading floods and so reclaiming it's flood plane.
20 years after man
The Thames Flood Barrier becomes very corroded and begins to fail due to rust and frost cracking the cement.
25 years after man
Most ordinary residential and working building have collapsed. 'When the leeves fail, Floodwater from the Thames damages the Wembely stadium and the pitch becomes a bog.
40 years after man
The wet and rotting tower blocks starts to deteriorate badly.
45 years after man
Wild packs of cats, dogs, wolves and ex-zoo predators like Hyenas roam the ruinous street and take over it's wild places.
50 years after man
The Thames Flood Barrier finally collapses in to it's self. London's open spaces are now forests. The steel is rusting badly, corrosion is rife and frost damage is also taking a heavy toll; as bridges and offices face collapse. The flooded London Underground tunnels collapse in on there selves along with the streets above.
55 years after man
The River Thames short cuts through the lakes that were formed by the former London Docklands and undermines remaining dock foundations causing their collapse.
60 years after man
Greenwich Peninsular and the Isle of Dogs become permanently flooded as remaining dock foundations collapse. The Thames floods floods as it reclaims some of its flood plane. River Medway reclaiming some of its flood plains.
75-100 years after man
The remaining big buildings and offices are in the state of collapse.
95 to 125 years after man
The Isle of Dogs, the Greenwich peninsular, Turrock, Greys, Dartford, Rotherhithe, Beckton, Canada Warf and the relevant stretch the River Thames become a muddy, radioactive swamp land with variose rivers, lakes and piles of rubble in it by the millennium. Other places like Moorefields, Lambeth, Battersea, Barns, Brentford, Putney and Wandle park become marshland once more. The remaining big building and offices are mostly collapsed by now.
2,000 years after man
London begins to turn swampy on mass.
10,000 years after man
Finally, after 10,000 years without maintenance, the Tower of London's towers fall inside.
1,000,000 years after man
London is now the marshland like the one the Romans had found during their conquest of England.