Thanks for your replies, i'm getting that it's a bad idea so I won't do that.
The room is a garden office. The internal sounder idea is an afterthought, I figured that it would make it undesirable for a potential intruder to stick around if there's an internal sounder.
I don't have the option of running any new cable to it, however I do have some CAT6 solid core cables also going to the room, could I use one of those for the sounder?
Or is that still a bad idea?
Mine is a plastic panel so it would be grade 2 at best, but i'm not a professional installer so I don't think grading counts for much. That said I would like to keep it as close to best practice/standards (despite my initial post!)