I am by no means an expert, but most UV torches emit light in the longer and comparatively safer UV-A wavelength which would register considerably lower on the UV scale than natural daylight.
If you check the datasheet section 2.4 page 15: https://www.silabs.com/documents/public/data-sheets/Si1145-46-47.pdf
You will see a graph relating UV index weighting to wavelength.
Since this sensor is reporting UV index and not a range of intensities by UV wavelength you might find that any other source of UV would occlude your torch- since the known effects of the wavelength of human skin are being used to derive a weighting that weights UV-A fairly low on the spectrum and UV-B much higher.
IE: Your UV-A torch might be very bright, but since its effect on human skin is negligible the weighting will cause all but the most high intensity of beams to register fairly low on the UV index.
You might have luck indoors, and you wont get a strong response since the torch is at the low-end of the scale, but you may get some response.