'Sussex gets a great deal of rain, and several rivers help drain off the excess'

Love this, J!

Nah, it doesn't really - in fact, Sussex, being about as far South as you can get in England, benefits from fairly mild conditions, and gets a lot less rain than, say, Scotland (hi Jo!) or Ireland (hi Rube!). Or Manchester (hi dody!) at that. A very notable exception was during the floods a couple of years back, but then it was very wet all over the country.

I thought that saying a place "is drained by a number of rivers [going wherever]" or "is drained primarily by the river Ouse [or whatever]" was just correct geographical-type terminology; and I assumed this construction to be global. Is that not the case, then?

"the Saxon Ella"--made me think of salmonella!

The Salmon Ella was, of course, a warrior king of the Salmon that, with some cronies, swam up river and laid waste the spawning grounds of other fish. These other fish found his presence hard to stomach.

More on the Saxon Ella here:
http://63.1911encyclopedia.org/E/EL/ELLA.htm
Not the sort of bloke to pick a fight with, by the sounds of it!

is this where the white cliffs of Dover are?
They're in Kent (and Dover is, incidentally, one of the Cinque Ports http://www.digiserve.com/peter/cinque.htm) - which takes us neatly back to where we started.

A lot of chalk makes up the South Coast of England. The South Downs (gentle rolling hills, slightly inland) run down into the sea (heading towards Kent), where they become eroded, and parts fall away, revealing the white chalk beneath. Thus White Cliffs.