CAP 413 Chapter 3 page 3 para 1.2.3.4 says:
"Pilots are expected to comply with ATC instructions as soon as they are issued. However, when a climb/descent is left to the discretion of the pilot, the words ‘when ready’ shall be used; in these circumstances the pilot will report ‘leaving’ his present level."
So the answer is yes, you "will" report leaving.
