You’ve asked about UV in the title then asked about “radiation” in the text...which is a bit confusing...
1. You are more exposed to ionising “radiation”, as in the sort of stuff the sun chucks out in Solar Flares and The Cosmic rays from deep space at high latitudes (specifically high magnetic latitudes), I suspect that’s why your company is metering your flights at high latitude...but that’s a different “animal” to UV.
2. UV is the electromagnetic radiation (with a wavelength just below that which is visible to the naked eye) ..that produces sunburn and your exposure to that is more dependant on the elevation of the sun, any cloud cover, thickness of the ozone layer, windscreen thickness and materials..probably plus lots more
To answer your question about Oz and being 20 south... well at those latitudes you are well away from the thin bit of the earths’s magnetic field around the poles and so are well protected from ionising radiation, so I wouldn’t be unduly worried about ionising radiation in flight...OTOH you are near the equator and I would however be more tad a bit cautious about the UV at ground level, e.g walking around without sunblock or any other form of UV protection whilst on a slip in that part of the world.