Color me naive but 400 bloody pounds for the SW then more for the charts
seems a bit expensive when one can buy equivalent land navigation SW for
$39(US) that includes maps.

The difference is:
They sell hundret thousands of copies of land navigation software to
individual users, additionally the digital maps are used by millions of
internet users on dozens of sites.
Just compare the number car-owners, motorbike-owners or even shoe-owners
(they are all potential buyers of land based navigation) to the number of
boat-owners and you get the picture.

Besides: A street map won't get you anywhere with your boat except when
trailering ;o)