"I think UV is used on the water on the way in to kill them off so the membrane doesn't become contaminated. Ignores the fact that UK tap water is chlorinated so sterile anyway."
Ah yes but what is the first filter in an RO system?
Carbon, which is there to remove the chlorine, since chlorine damages the RO. That is why you get green gunk growing on the fluffy sediment filter, because there's no chlorine to kill the stuff.