I read somewhere that Airbus pilots arm the LOC switch first and then arm the APR switch once the LLZ is intercepted. My information may be incorrect. But this is what I do.
On flightsim, you are cleared for the approach, that's it. You are then asked to contact the tower. So, LLZ and G/S interception take place between the time approach clears you and once you are fully established on the LLZ and G/S, then tower says: "cleared to land". You are not cleared separately for the LLZ or G/S on flightsim.
I have not come across ATC saying: "cleared to decend on the G/S". I used to listen to a/c chatter a lot, one of the things I've heard go something like this: "Kingfisher 316, you are 23 miles north-west, cleared for ILS approach RWY27, turn right heading 250 to intercept localiser, maintain 3,700 until established on the localiser, contact tower on 118.1, report established."