Being close to the front depends on what happens in Cisalpine Gaul, I'd think.
To be sure, as a port city, it is on the "front" for any power that can make a strong fleet to attack with.
What's all this talk of "England," BTW? The OTL migrations that led to former Britain having a big region on it called "Angle-Land" are a good 600 years in the future, and I doubt the ancestors of the Angles and Saxons are even on the North Sea shore yet, they are probably in Scandinavia at this time. That big island across from Gaul is called Britain, or rather since that is the Roman rendering of it, whatever Pytheas referred to it as. Quoting Wikipedia on the subject of that explorer, "According to Strabo, Pytheas referred to Britain as Bretannikē, which is treated a feminine noun." Presumably he met mainly with "P-Celts" such as the Britons known later as Welsh, and not with "Q-celts" such as Irish or Picts. In P-Celtic British languages the isle of Britain would be something like modern Welsh "Ynys Prydein."
I don't know if we have any sources at all on what the Carthaginians called the island or its peoples.
By the way, the treaty mandate that they cease their Atlantic trade activities is a hell of a sweeping and high-handed demand! I wonder how well Carthage will comply? Doing so means folding up their tents in the north and cutting their former trade partners off for no local reason. How exactly is the Alliance to enforce it anyway?
I suppose the mechanism is, the Massaloite sponsored traders will react violently to any Carthaginian activity they observe in the north, and cite the treaty in any negotiations.
But the flip side of it is, the Carthaginians have been cultivating trade relations with the various northern peoples for a long time, and their partners are familiar with them and liable to favor them over the Greek upstarts.
Time and again I've attempted to explain my misgivings at the apparent one-sidedness of the Hellenization of Massaliote south Gaul. I wrote at some length something using a possible scenario for the expansion of Massalian power into Cisalpine Gaul, in the context of the recent war. But I didn't get it finished in time and later posts have mooted its direct relevance, so it lies unposted.
It is a valid choice of the author to assume that the Hellenes of the League territory will assume that culture and civilization equal Greek culture and civilization, and disdain to learn from and adapt to the Celts within their network who have chosen, whether out of attraction or fear, to associate with them rather than ally against them. What I'm trying to say is, that the more the Hellenization of Gauls is a matter of fear, coercion and subordination, the harder it will be for the League to achieve the Manifest Destiny many of us readers and I suppose the author are eager to see--that is, the spread of Massalian hegemony north into Gaul, in time assimilating it and much of Britain as well. And if one looks at the broader "Celtic" zone as indicated on a recent map, yet other prospects open up. I wondered for instance if, after assimilating Cisalpine Gaul, which seems to be coeval with the Po River valley, they might leap across the north tip of the Adriatic into Istria, found some city (or take over one) at or near Trieste, and then maps seem to indicate a pass nearby where they could enter the Danube valley in OTL Austria and Hungary. Well by gosh, it seems this zone is Celtic too at this date!
It sure would be interesting. But we have seen very little sign of the Hellenes of the League adapting to the local Celts they have lived among for a century or more; little sign that allied Celtic tribes enjoy status and influence in the leading councils of the League. If in fact the League were an alliance of Celts and Greeks, then their ability to move rapidly into this zone, and win over the allegiance of northern Celts accustomed hitherto to trading with Carthage, and assimilate Cisalpine Gaul and all that would be eased.
If instead, as it seems to me to be the case thus far, they carry with them an overbearing Hellenic chauvinism that assumes Celts are barbarians to be led like children to actual civilization, and have no Celts among their leaders to demonstrate the advantage of association to more independent other Celts...then the Greeks can expect only tough sledding. The process can take the form of subordination and assimilation by cultural transformation to a near-purely Greek model, but it must be slower, for they seem likely to sooner or later infuriate the people they hope to profit from. And the large number of Celts who are deep within the League might be persuaded to rise up if they do not feel like first-class citizens, and considering how many references have been made to Celtic soldiers in Massaliote armies, a Social War of that type might be the undoing of the whole League.
I keep thinking back on the decision to bypass the established Gaulish town of Burdigala and instead establish an inferior rival port town to its south, a decision that makes sense only if the Massaliote plan is to steal the Gaulish town's trade. If they had learned lessons enabling them to interface with rather than bypass established Gaulish communities, setting up shop in Burdigala instead would have been the wisest move; doing this economically irrational thing instead suggests hostility to me, and I think the Gauls will view it that way too.