Sailormed is on target by suggesting that you just piggyback off of the pressure pump's switch. Using the same switch will provide proof that the UV system is getting power since your water will also be off if it isn't. LED's draw very little power, I wouldn't worry about the need for relays. Just wire it up to the pump
side of the pressure switch.
I always questioned whether water flowing past a UV lamp has enough exposure time to be effective. In a contained system where the water is constantly flowing, the water gets many passes by the lamp, so it aggregates the exposure, adding up over all the passes and may be effective for the system as a whole.
If you are just exposing the water to UV for just the brief time it is passing the LED in a pipe, would this be enough time to be effective? I think of it like tanning. You can be exposed to the sun for short periods with no noticeable effect. It is the long duration of exposure that kills the skin cells. I would think that in order to kill any biologics, you'd need more dwell time or multiple passes.