Oh dear, one loses the will to live...
Regarding "ready for immediate takeoff", this should not be used because the only time a
pilot is allowed to use the word "takeoff" is in reply to
his takeoff clearance.
This was one of the results, I think, of the famous Tenerife 2-airliner runway collision in the 1970s.
The purpose is to prevent some unrelated and hot under the collar pilot hearing the lone word "takeoff" (e.g. as a result of a garbled transmission) and putting 2 and 2 together to get 5 and thinking it was
his takeoff clearance, and entering the runway...
However, saying "Nxxxx ready for immediate" (without speaking the word "takeoff") is fine and is widely used commercially too. It is appropriate in certain situations; examples:
(1) There is an aircraft on final but still miles away, the ATCO doesn't know you** and this lets him know that you are ready to get off quick
(2) You have two runway entrances, with a long queue for the furthest one, with a student (or a low currency pilot) bogged down at the head of the queue, and you are all ready to go right and happen to be next to the other entrance, and there is plenty of runway available to depart from there
(3) The ATCO is obviously incompetent (go to Valencia, among many examples in southern Europe) and this is a way hopefully doing something about the situation. How well it works probably depends on who else in on duty; in Spain it may have the opposite effect, and Ryanair/Easyjet tend to use it to let off steam
**This is never spoken overtly but ATC do give priority to pilots who they know are not going to give them trouble. For example somebody who is obviously having trouble with the radio and other stuff is unlikely to be given a "land after". The ATC at a given airfield also know which of the locals are OK to be asked to orbit on base leg, etc.