This subject was flogged to death in Nov/Dec 2002 !!
search 'CDA' and it's all still available to you.
Basically the rules in the London TMA below TA (6000ft), require that no segment of flight is flown 'Level', defined as a change in altitude of less than 50ft in 2nm. Therefore use whatever ROD will achieve that aim. Note this is 'monitored' and letters sent to the companies of the 'sinners' !!
The requirement for a min. ROD = 500fpm en-route, in the hold & down to TA is still mandatory.
Moley.