I have always been led to believe that D&D will be happy to oblige if a GA aircraft calls requesting a practice pan (not a Maday as has been suggested), providing there isn't a real emergency in progress. And you can avoid stepping on such transmissions by observing basic RT etiquette;
Listen before you transmit
Make the initial call short.
London Centre G-xxxx request practice pan
Heres what the CAA have to say:
http://http://www.caa.co.uk/docs/33/fod200608.pdf