In reply to Steve nevers: It will cost a little at first to start... membership with the BMC, then registering with for the award you're planning to do with the MTA and then the training of course, which does vary... worth shopping around between climbing centres and MIA instructors here.
Once you've done your CWA training, contact a number of walls and explain you've done your training and now looking to shadow sessions to gain valuable experience. Most climbing centers will be willing to help and offer you sessions you can help assist on. Be willing to travel mind.
Make sure you assist at a number of different centres for experience as some walls do have different numbers per group and some taster / birthday sessions do vary and the more varied sessions you do the more confident you'll feel and ready for your assessment.
Hope this helps and good luck!