No Benjbh, not in the way you read it.
You harbour a desire to go RW. Once you have reached a particular stage of training will they assess your aptitude for FJ, RW or ME. If every thing else is equal they may take your preference into account.
However whatever your aptitude if there are no empty seats then you would not be streamed that way. More likely, if there are empty seats on one of the other streams then that is the way they would stream you assuming that your aptitude was acceptable.
There was a time when some trainees attempted to fiddle the system so they could go to trucks as a short cut to a CPL/ATPL. It didn't always work. Not suggesting that is what you have in mind but just to show that there are no certainties.