I was told not to enter CAS and that is a mandatory readback
It is?
I looked again at CAP413 (as your location is listed as "UK" and you speak of Solent) - readback items;
Taxi Instructions (not one of them)
Level Instructions (not one of them either)
Heading Instructions (nope)
Speed Instructions (nope)
Airways or Route Clearances (not quite - but closer)
Approach Clearances (nah)
Runway In Use (nope)
Runway Actions (as it used to be phrased - but "nope)
Transponder Operations (nowhere near)
Altimeter Settings (nope)
V.D.F. Information (nope)
Types of Radar Service (almost, just a few minutes later - but nah)
Transition Altitudes (nope)
Frequency Changes (also nope).
Now, had you been given a "positive" clearance to *DO* something then I could see it coming under item five in that list - but getting a "negative" to *NOT* do something.....which bit does that come under?
Why not "Wilco" (I will comply with your instruction not to enter the airspace - but as it's not on the readback list I won't waste your time with blocking the frequency)?
"Wilco (callsign)" Shorter, sweeter and in compliance with the manual?