I hope this isn't all "pot half empty" pessimism. But I think that's the reason why you may have had a problem? A person responding with a quasi-military or paramilitary background (which the old RUC that became the PSNI could be argued to have) might see a trigger unit as being the complete "gubbins" springs, hammer, sears, trips and etc. and fear that it was wanted to complete a "bits" project involving something not at all like what you are wanting to discuss.
So, yes, I can see their side of the enquiry even though the thing itself, as on this Browning just a single bit of mateal, or Beretta just a collection of bits of metal. But they may be thinking AR15? So yes here in Great Britain not, per se, subject to control. As without specifying the exact make and model of firearms an imaginative mind could be concerned that this was a trigger unit for an altogether different thing to what you are asking about.
As for better or worse the de-activation specification on most all firearms now involves destruction or despoilement of the de-activated items trigger unit. Without further information the person answering you enquiry may have that to mind. I think a fuller follow up might get a different answer?
Browning trigger change - just a trigger
Beretta - a unit that drops in and out
AR15 - what might cause concern to PSNI