Although the term "Folk Art" works pretty well for the US, it's likely that what you're looking for is categorized differently for other cultures. In the US, there were (and are) settlements of immigrants who came together, stayed - for a time - together, and thereby "segregated" styles that, while they existed in the "homelands," were merged sufficiently there to be less noted. "British" also has a longer "industrial" tradition that probably buries the sort of "primitive" stuff you want amongst the "commercially manufactured collectibles."

While it's likely not the sort of thing you're looking for, I've noted that British artists (or Frenchmen painting to order for British clients) prior to the mid 20th century had a very distinctive (and IMO "peculiar") way of drawing/painting horses, with scrawny long necks and pointy noses so that they looked like long-legged wolverines.

For "primitive paintings," and especially for symbols/signs, perhaps a search for architectural styles from a particular time and place might be a way of finding how houses and barns etc. were decorated in distinctive sytles. I would suspect that "British" is too broad a category to be too revealing, but a more specific district/region within the British realms might get more helpful results.

Symbolisms often are revealed in "advertising" if you can pin down a place and time, or a style of ads like patent medicines or farm implements that provide a few hits. Cigar boxes often had rich symbolisms in the US, although I don't know how common that was elsewhere.

In the US, there are a few - but pretty good - resources that crop up in a search for "Erotic Folk Art," (tame by current standards, but some of it is pretty cute).

If the first search term you think of doesn't work, you'll need to come up with a different set of terms that's in better agreement with what the thing is called by the people who own the secrets you want. Finding collectors of what you want would likely be most fruitful, especially if they've got an "organization."

John