Wow - this really got out of hand quickly.
Essentially there are two options here either do it or don’t - judge each on their merit.
Out of interest Roestalker, if the question had been ‘am I allowed to get blind drunk and drive home?’ would your advice be, no because it’s illegal or would you say ‘absolutely fine because I know someone who does it and what’s the likelihood of being caught - anyone who says otherwise is a complete idiot and we shouldn’t be letting anyone tell us what to do’?
Annoyingly, the AIHTS is a European fur trapping legislation so our use is an unintended consequence. The requirement is for the target animal to die in (I think) 90 seconds or less and fenns don’t always do that according to the scientists. When you see one of the new options actually go off - the difference is quite clear.
To clarify - you can use fenns for rats and squirrels perfectly legally but you are running a risk of you catch a stoat (and are caught).