I've been thinking about this some more. I've been looking at the map and
consulting Strabo on the ports used to sail to Britain. He says (writing
after AD 23):
" There are only four passages which are habitually used in crossing from
the mainland to the island, those which begin at the mouths of the rivers -
the Rhenus, the Sequana, the Liger, and the Garumna. However, the people who
put to sea from the regions that are near the Rhenus make the voyage, not
from the mouths themselves, but from the coast of those Morini who have a
common boundary with the Menapii. (On their coast, also, is Itium, which the
Deified Caesar used as a naval station when he set sail for the island."
The Garumna (Garonne, comes out at Bordeaux) and the Liger (Loire, Nantes)
are not an option for an invasion fleet, as they would involve sailing all
the way round Britanny. The Sequana (Seine) comes out at Le Havre and would
be a good port to reach the Solent from.
Ships leaving the Rhine area would sail down the coast to Bruges, Ostend or
Dunkirk before setting out to sea, and, sailing more or less west, would
probably be heading for Dover, Richborough or the Thames Estuary.
Itium is Caesar's Portus Itius, which may be Boulogne (Gesoriacum), although
T. Rice Holmes apparently thought it might be Wissant, a little further
north-east, which has a bigger harbour. Strabo seems to be saying it wasn't
much used at this point (the "only four passages habitually used" being the
four rivers mentioned), although a lot can happen in 20 years, and we know
Claudius embarked at Gesoriacum.
Sailing direct from the mouth of the Rhine is a long trip, probably too long
for our purposes. However, it seems that, as far as the merchants in the
early 1st century were concerned, sailing west from the Rhine, via the coast
of Belgium or Normandy, was a better crossing to Britain than north from
Boulogne. Perhaps this was Plautius's westward voyage? And perhaps if a
secondary force targeted the Chichester area, it sailed from Le Havre?
Any nautical types on the list who might know the relative merits of the