Sea levels rose by 20 cm during the last century, but the effects are only
seen during exceptional high tides, which are infrequent. Moreover, a
one foot rise in sea level can easily be held by building embankments.

Not easily, even if the money were available, which it normally isn't.

The level of Penzance prom was raised following the Good Friday 1962 storm,
which caused extensive flooding.
Of course, what actually happens is the sea hits the prom, shoots 100 feet
in the air (regularly, I've got pictures to show it) and floods the lower
area behind. This has subsequently been offset to some extent by valves in
the storm drains etc.

As well as sea level rising, Cornwall is sinking - If you ever fly into
Scilly on a clear day you can clearly sea the old field hedges (granite
walls to the English) extending across the sand & under the sea.

Raising the level of the sea defences can make things worse, by trapping so
much water behind, and if they are breached - well New Orleans here we come.

On the plus side 8 -10 foot swell today, shame it's so cold.

Graham