The Vegan Society (UK) was founded in 1944 -- that much is easy to discover on their website. But I've heard that the goals of the Vegan Society have changed and evolved over time as different people led the organization. This information is harder to find. What were the original goals of this society, and how do they compare to the present-day goals?
As you said, the original society still exists as The Vegan Society, and they were formed by strict vegetarians that went on to exclude all animal products. According to their origin story the group etched out a definition of veganism. Vegan outreach by the society allowed them to absorb vegan movements from other locations. Vegans aren't all bound by the Society, membership isn't a necessity, but many vegans do know the definition of veganism that was crafted in those early years.
According to Wren, the original society had more moral interests.3
Watson and his predecessors felt sure that the way to humanity’s heart was through its stomach, and they leveraged veganism in hopes of revolutionizing social relations.3
However, this was in contrast to the radical hopes that underpinned the group.
Whether countercuisiniers or moral entrepreneurs, the vegan pioneers carried a radical agenda to challenge the power structures manifest in the state, science, industrialized agriculture, and the Nonhuman Animal rights movement.3
So, the vegan movement was forged with many radical interests that caused factional differences through the years. One notable problem is corporate interest in appropriating the movement. Over the years many vegans have converged on opportunities to exploit their veganism as a means to success.
Whether the original society equally used veganism unethically as a means instead of an ends might also be debatable. Watson seemed to be a thoroughly genuine character who was willing to undergo great hardship for his cause.4 During WWII he was a conscientious objector because of the militaries abuse of animals. Watson scraped through those years on his savings, and a small amount of welfare. An outsider implying unethical behaviour or overly radical objectives has a probability of logic flawed by conflation and conjecture.
Veganism is radical by nature of its existence as a counterculture alone. Any vegan that draws radical attention can be claimed to be an attention seeker. Each successive activity of vegans could be confused as the emergence of a new radical vegan faction. No doubt the original style of Watson's people doesn't suit all vegans, but that doesn't rule out the original interests of the early breakaway society. Veganism is equally defined by breaking new ground.
See Wren's article and sources for more analysis of the factional changes.