(Not arguing, just adding some perspective.)

Penne is pronounced "pennay" in the UK in my experience, making this pun kinda dysfunctional.

Also as pointed out, UK cryptic crossword answers are phrases in actual use, not new word combinations.

Edit to add relevant example:

See 9 across ("Rotten people sin to produce malicious missive") here: https://www.fifteensquared.net/2021/03/08/independent-10733-...

I also noticed that "poisoned pen" isn't a phrase in British English, instead it's "poison pen" or more typically "poison pen letter".

Is it pronounced "pen" anywhere? That sounds very wrong to me.

Indeed... it seems that the consensus is tending towards this being so bad a pun that it isn't even a pun :)

It's a poisoned punne.

Leads right into a clue hinting at Canadian origins, a la "poisoned pen, eh?"

So in the UK 'poisoned spaghetti' would not be acceptable as a cryptic crossword answer, because it's not a _recognisable_ standalone phrase, even if it's a thing that could exist.

It doesn't make sense outside of the pun. "Poisoned spaghetti" makes as much sense as "poisoned penne", in those terms.