As a regular into Gatwick it's pretty standard for us to plan the RNAV as a backup every time.
It's the same final app track and descent angle, it's basically the ILS with no ground equipment, but to a higher minima 580ft DA (26L) or 700ft (iirc for 08R) rather than 400ft CAT I.
Company SOPs may vary, and aircraft equipment, but it's not much of an issue. Takes us about 30 seconds to brief the RNAV whilst doing the approach brief, but with only 3 A380 rotations a day, there are not many times you have to do it or wait.
If the weather is good enough, you could always ask for vectors to an x-mile visual instead of an RNAV... you still have the height/distance on the ILS chart to compare, although don't use the LOC/GS signals as they can be very affected.