SEVEN METERS!? Wow!
The original marshlands were drained gradually for agriculture, and the land sank as it dried. The southeast of the island has been sinking relative to sea level for natural reasons as well.
From the link above:
> The Embanking of the tidal Thames is the historical process by which the lower River Thames, at one time a shallow waterway, perhaps five times broader than today, winding through malarious marshlands, has been transformed by human intervention into a deep, narrow tidal canal flowing between solid artificial walls, and restrained by these at high tide.
> With small beginnings in Roman Londinium, it was pursued more vigorously in the Middle Ages. Mostly it was achieved by farmers reclaiming marshland and building protective embankments or, in London, frontagers pushing out into the stream to get more riverfront property. Today, over 200 miles of walls line the river's banks from Teddington down to its mouth in the North Sea; they defend a tidal flood plain where 1.25 million people work and live. Much of present-day London is recovered marshland: considerable parts lie below high water mark.
https://en.wikipedia.org/wiki/Brighton_and_Rottingdean_Seash...
" The single car used on the railway was a 45 by 22 ft (13.7 by 6.7 m) pier-like building which stood on four 23 ft (7.0 m)-long legs."
See the river Severn, whos estuary tidal range is 15 metres, and the second highest in the world.