Good point actually, don't have the right answer, but can tell what happened to me four years ago in Gatwick (ok, it's not the same but pretty close
); on the approach, just upon ILS intercept, with the app mode armed and vor/loc and g/s captured with a/p on we both noticed the airplane symbol on the map display going far away from the magenta line...asked tower about ILS status, the controller wait a bit before answer than he replied "sorry, ILS on test (or something like that and no notam) and not working, can you accept radar approach?", we were in the cloud so we couldn't see anything, accepted radar approach and fortunately break out at 800 feet and land...so, Fathom, don't know how often that happens, but for sure it happens, so you'd better be prepared.
FB