Hello everybody,
First in Spain we apply EASA and ICAO rules, maybe in FAA or in UK has something slightly different.
FROM ICAO, clear for the app means that you are clear for both lateral a vertical navigation but you can start your descend if you are within 5 degrees or half of the max LOC desvistion (I'm saying this by memory so the wording could be different).
Also, if the ATC want you to maintain the altitude way after the loc interception they should say it like they do it Le Bourget, they clear you for the "LOC only" maintain the altitude until you are advised by the ATC that you can start your final descent.
If you are expecting to achieve the LOC too high you should request descend to the ATC.
This is my humble opinion because I haven't seen anything written telling you that you can leave the altitude assign by the ATC until established.