The Boojum distribution is more like Shadowrun than Earthdawn, in that there are only the four meta races on Earth. There have been no obsidimen or windlings seen as of yet, and there have been no Saurids to migrate to Earth. Approximately ten percent of the population transformed, or will transform as the mana level in remote areas rises, with the numbers evenly divided between elves, dwarves, snarks, and trolls. As pointed out, only about half of the trolls survived, due to the difficulties of transformation. When a base stock human passes through the Rabbit Hole, there's a chance of their transforming, but the odds of it happening vary wildly. You might have a train where nobody transforms, followed immediately by one where half the passengers are changed. The Rabbit Hole environment should never be predictable. You are correct that most people do not know, and most do not care, about the actual percentages and the chance of LGF. There's a Galvanic researcher in the London book who's done some serious work on determining this, and finding the actual reason for LGF, but his work is over the heads of the populace, and they might not care anyway, as his work contradicts the malicious interpretation of Darwin that the bigots are using.
It's worth noting that the mana field of Earth has not stabilized. It's still got thin spots. It may be a couple of years before Earth achieves a global average mana level equal to that of the Gruv. There may still be thin spots, such as the Takla Makan or Death Valley. See the Astral Space materials in the Players and GM's Guides. People living in the Takla Makan region are going to be highly unlikely to go through LGF simply because of the low mana level in the region. If they travel to the great bamboo forest of the south, however, their chance of transformation would rise due to the higher mana field of the region.
If a decision is made to transform a player character, I would recommend discussing it with the players first, and making a decision as a group whether to select a character, or roll dice for each, or some other method of determining who transforms, if at all. I would also suggest that the player have input as to what metarace their character becomes, as this will strongly impact playability. If you just roll dice, you could end up with a character than can no longer follow their profession - an Airship Pilot becoming a troll, for example.
There is a social aspect, but it has to do more with bigotry than genetics. Some upper class families will not allow their snark or troll relatives to be seen in public, ashamed of having such coarse breeding turning up in their line. Some lower class families may ostracize an elf, as someone who has taken on airs and is parading around like they're a toff. The Boojum Aid Society exists to assist such persons, who have been cut off or cast out from their families.
The number of snarks in the population will rise the fastest from reproduction, as, like Earthdawn orks, multiple births are the norm. Elves are just not very fertile, as befits a race that lives 300 years. This will of course contribute to the social pressure from bigotry, as the lower classes are already considered (by some of the nastier aristos) to breed like rats.
You can find the 1881 UK Census data online at
https://ukcensusonline.com/census/1881/. What I would suggest, though, is that you have as many Boojums as your game requires. If your interpretation of
1879 has more Boojums, then have more Boojums. If your game requires more base stock humans, have more base stock humans.
This may be more than you bargained for
. Hope it helps.