We got one of these recently:-
https://cpc.farnell.com/pro-elec/pe00101/insect-killer-12w/dp/SI15607It does seem to work quite well. We hear the buzz of a fly getting zapped a few times are day. There are dead flies in the tray. There seem to be quite a few around it as well, so I would put it above a cupboard like we've got it or somewhere else where falling ex-insects won't matter.
If a fly sticks to the bars, it gets incinerated with the smell of burning hair.
Ours did have an electrical buzz all the time, and I found that the choke for the UV tubes was only held down with 2 screws even though it had 4 mounting holes. 2 more nuts and bolts fixed that.
(In doing that, I did notice that the whole thing runs quite hot, which I think was due to the fly killer being made for 230 V and the actual mains being over 240 V here. I geeked out and repurposed an old christmas light transformer to drop the voltage to about 210 V and it runs a bit cooler. In doing that, I've probably save enough electricity, over the life of the fly killer, to pay for about 1/10th of the time that the modification took, at minimum wage).