From the UK AIP:
2.2 Minimum Rates of Climb and Descent
2.2.1 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.
2.2.2 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.
My reading of this is that flying at less than 500fpm is not prohibited, as long as you have informed ATC. The requirement to inform ATC is removed when flying below Trans Alt, as per 2.2.2. Therefore my conclusion is that one is perfectly entitled to fly at less than 500fpm below the Trans Alt whenever it is required. The "Note" in 2.2.2 would seem to reinforce this, as a CDA is certainly an "operating requirement" at some airports.
I have also come across a document called "Noise from Arriving Aircraft - An Industry Code of Practice", dated Sept 2001, which has the following line:
35. The instructions in the UK AIP that referred to a minimum descent rate of 500 fpm
are to be clarified, enabling extended descents in line with CDA to be achieved
without need to notify ATC of lower rate of descent below transition altitude. This
will come into effect from 4 October 2001.
I think note 2.2.2 from the AIP is this clarification. Admittedly it's not overly clear that it is applying to CDA's, but I think a literal reading of it allows for less than 500fpm when required.