I appreciate what you're saying Sir, and I dont doubt your knowledge. I am speaking from personal experience of what i have seen.
In some circumstances, they WILL take into consideration your request to fly a certain aircraft. Im guessing that if you're skills and aptitude suggest that you would be suited to all roles, and if they had vacancies in all areas then your request would, and IS honoured.
The same applies for actual postings. They ask you where you want to go, 9/10 times they wont actually listen to what you say, except some times they do.
I've a good friend whos an ATCO in the RAF. He did 2 tours up at Prestwick, come the 3rd tour, they wanted him down in Leeming, he asked if he could say up north for a little more time as he was doing some courses outside of the RAF and they honoured his request. He is now, 2 years later than planned, based at Leeming.