FWD, the UK authorities have decreed that if you wish to sit the CPL Skill Test for the issue of a UK CPL then you must have passed the UK Radio Telephony Operators Licence before you sit the skill test.
There is no need to actually HOLD the licence at the time and, as RT licence issue is free of charge *IF IT FORMS PART OF A UK PILOT LICENCE*, there is nothing to gain from applying for it in advance. Indeed, you will *LOSE* the RT application fee (as an early application for an RT licence NOT forming part of a UK pilots licence will cost you).
Other JAA member states don't necessarily insist in the same way. The UK is just trying to keep up the standards of RT (and, for once, I applaud them for it).