Is “ruddy” an actual Briticism?

It is definitely a British expression, an old fashion one:

Ruddy:

As a British slang euphemism for bloody (q.v.), first recorded 1914. (Etymonline)

Ruddy (adjective):

• Ruddy is used by some people to emphasize what they are saying, especially when they are angry. [British , old-fashioned , rude , emphasis]
  • He took the paraffin stove on a picnic and the ruddy thing wouldn't work.

(Collins Dictionary)

Ruddy (adverb):

• British Slang (adverb) He'd ruddy well better be there.

(Dictionary.com)

From: Pelham Grenville Wodehouse “chapter XVIII and XX”, in Jeeves in the Offing, London: Herbert Jenkins, 1960:

• “Of all the damn silly fatheaded things!” she vociferated, if that's the word. “With a million ruddy names to choose from, these ruddy Creams call one ruddy son Wilbert and the other ruddy son Wilfred, and both these ruddy sons are known as Willie. Just going out of their way to mislead the innocent bystander. You'd think people would have more consideration.”

(Wiktionary)

My British-born mother (now 77) uses it quite often, much to my amusement. She would've gotten it from her mother, an Italian Catholic immigrant to Southern England in the late 1920s. She would've minced her oaths as she learned English, I imagine. What makes it more amusing is the extra emphasis put on the minced word, as if to compensate for mincing it!