If non rnav a/c following a380 then a 15 mile gap is required to safeguard the ils. Otherwise to mitigate, the tower asks for an 8 mile gap behind the a380 with either 2 or 3 rnav arrivals behind depending on current gaps. This allows the runway to be utilised better. Gaps and rnav requests are usually made when the a380 first appears on the towers radar roughly 20 mins before landing so aircraft being asked to fly rnav arrivals usually have notice a reasonable amount of time before the start of approach. If you dont want to fly it you can always ask for an ils approach but that may incur a slight delay if other a/c will fly rnav.