Frankly, although I did win my third attempt at this map, I relied significantly on map knowledge and a bit of luck, so this would possibly not be the optimal approach without map knowledge.
Now IMHO the problem on this map is threefold:
First, although lots of land is available, Justinian and to a lesser extent HC will expand into this land rather sooner than later.
Second, the three neighbours are all likely to get an early religion, and especially Charly and Justi are military threats.
Third, but least relevant, HC has a propensity to settle his second city close by, found a religion, build some winders and grab London's second ring.
Settling London further east to discourage HC is not worth it, due to moving away from the food.
In the east, two cities would be sufficient to cut Justi off: one 4W of London, sharing the corn and pigs, and the other one SE of the clams. Both these spots are sufficiently good to be viable cities, and they need to be because in my plan these are cities 2 and 3.
In my first game, I settled my second city in the center and Justi made cutting him off impossible, forcing me to plant the clams city one tile further north and proceeded to funnel settlers north... That would have been manageable (pults), but I was playing too quickly and got attacked ~ T70 with nothing better than warriors, so that was it.
This is a mitigation to problem 1. This can the be followed up by settling north, claiming space to backfill a city or two, resulting in at least 7 cities (London, two cutting of Justi, three cutting of HC and one backfill). This is sufficient to go for Cuirs.
Now of the three problems, the first one has been solced, the second is out of my hands, but I can still meddle with the third one

:
My idea was to send the starting warrior towards HC's likely second city spot, get there before him, and DoW him, expecting that he would not fight my warrior with his archer escort and settle further away. Obviously there would be diplomatic ramifications, but only with HC, as we would probably not know anyone else at that point, so no "you DoWed our friends". That would make him my enemy, but allies would still be available in Justi and Charly. Even with the -3 HC would likely not be furious once the war ends, so the only relevant change to adopting another religion later on would be the absence of open borders. Anyway, HC is less of a threat than Charly or Justi, especially if he goes off to build wonders, so I was not that worried about the long term impact. He would be my first target anyways.
Unfortunately I was unable to put this into practice, as my warrior must have scared him sufficiently without a war, as he ended up settling 1N of the corn. That is sufficiently far away that cultural pressure should be manageable.
Now, my second game, I proceeded to fogbust the north, which was a big mistake as I was not fogbusting the west where I wanted to settle and a barb city popped up early, so on to game three.
My tech path was Agri -> AH -> BW -> Pottery. With a bit of luck (i.e. AIs researching AH), it comes in just in time for the worker not to loose any turns.
London then got a warrior out while growing, and then two settlers went west, settling on T37 and T44. I did not fogbust the south at all, as there were enough AI units handling barbs there.
Now I got lucky, and only Charly and HC founded early religions, with Justi adopting Charly's, resolving problem #2 from above. I obviously adopted that religion as well.
In the meantime, HC built wonders, so I claimed a city 2E of London's cows and only cut off HC after that. With Fin and lots of rivers I went for the classic cuir rush, going Nationalism immediately after CS, because the AIs were sufficiently far behind, and Taj is nice (even though I missed the marble). I was a bit late in producing my units, so I stopped after capping HC, as people were starting to get too close to Rifling, and finished up with tanks.