Caesar tamen in Britanniam proficisci statuit:
Caesar, nevertheless, decides that they set out for Britain.
Correct, but strictly speaking there is no "they" here. "Caesar, however, decided to set out for Britain."
Britanni in omnibus fere Gallicis bellis auxilium hostibus nostris subministraverant.
The Britons supply help to nearly all our enemies in the Gallic War.
"In almost all of the Gallic wars the Britons had supplied help to our enemies."
Note that "fere" falls within the semantic unit "in omnibus Gallicis bellis," so the adverb naturally modifies this rather than "hostibus nostris." Also note that "bellis" is plural.
Parse the tense of the verb "subministraverant" and you'll see that it's pluperfect, which makes sense, because this is the historical reason (something "had happened") that prompted Caesar's decision.
Sed primo genus hominum, loca, portus, aditus cognoscere statuit:
haec omnia fere Gallis erant incognita.
But at first a race of people set out to ascertain places, a harbor, and a means of approach:
this is unknown to nearly all of the Gauls.
What is the subject of the first sentence? It's implicit, i.e., Caesar, the same subject of the previous "statuit." Despite its final -us ending, genus is a third declension neuter noun, going along with the other objects in the list. Now the sentence should make more sense: "But Caesar first set out to understand the type of people [i.e., the Britons], the locations, the ports, and the approaches." In the final clause, "fere" could be construed with either "omnia" ("almost all these things") or "Gallis" (as you've translated it). You've used the singular present "this is" when the verb is plural and past ("erant", "these things were").
Nemo enim, praeter mercatores, illo adiit:
neque iis ipsis quidquam praeter oram maritimam notum est.
For nobody, except the merchants, goes to it:
and not any except for they themselves recognized the seacoast.
Again, pay close attention to the form of the verb--"adiit" is the perfect (contracted form of "adi(v)it", not the present ("adit"); "Nobody, besides the merchants, went there".
The second clause explains the constraints on the knowledge of this group of Gauls. Start to break the sentence down when it trips you up; take an inventory of what you know about the sentence rather than attempting to force some meaning upon it after having looked up the vocab. "quidquam" could be neuter nominative, and the verb "notum est" requires this, so it's safe to say that this is your subject. Keep in mind that "notum est" is a passive construction, so it can't be an active verb like "they recognized". Add in "neque" and we have "And neither is anything known..." "iis ipsis" is a dative with notum, clearly referring back to the mercatores "to them [the merchants]". "Praeter" takes an accusative and immediately precedes "oram maritimam," so it has to mean "besides the seacoast." It can't go with "ipsis" or any other word given the syntax. Idcirco, "And neither is anything known to them except the seacoast."