Well let me amplify my response....
In the London TMA you will very rarely hear a 'when ready'.
Most instructions to climb or descend are to be followed when instructed to not when you (as a pilot are told to by the computer).
If coming into one of the holds you could expect to be given a 'level by' but we would still expect you descend at the min required (500fpm). OR ask if you can delay you descent for a specified distance.
From the AIP ENR 1.1.3
2.3 Minimum Rates of Climb and Descent
2.3.1 In order to ensure that controllers can accurately predict flight profiles to maintain standard vertical separation between aircraft, pilots of aircraft commencing a climb or descent in accordance with an ATC Clearance should inform the controller if they anticipate that their rate of climb or descent during the level change will be less than 500 ft per minute, or if at any time during such a climb or descent their vertical speed is, in fact, less than 500 ft per minute.
2.3.2 This requirement applies to both the en-route phase of flight and to terminal holding above Transition Altitude.
Note: This is not a prohibition on the use of rates of climb or descent of less than 500 ft per minute where necessary to comply with other operating requirements.
...often the clearance is actually an unspoken 'when ready', or there is some 'secretly' assumed descent rate or restriction in there somewhere.
so to from what you said i have shown you it is not an unspoken when ready and the secretly assumed descent rate is in the book.