Just for the record there is no actual prohibition on the use of rates of descent of less than 500 fpm when in the en-route or holding phase above the Transition Altitude.
The requirement is that the pilot should inform the controller if the rate of descent either is, or is anticipated to be, less than 500 fpm.
Incidentally just a word of caution to pilots operating into LHR.
If when below the Transition Altitude you are requested by the controller to report passing a particular altitude, this is almost always so that another aircraft in close lateral proximity can be descended to that altitude. If in order to carry out a continuous descent approach your rate of descent is less than 500 fpm then I think it would be prudent to inform the controller of that fact even though the requirement to do so has been removed.