When I went through as a student in 1973 there was no streaming. Everyone did the same course : Varsity (plotting) a few hours JP (low level visual Nav)/and Dominie (plotting faster, radar and a bit of low level). Depending on how you performed at each you were posted . The fast jet OCUs were, not surprisingly, unhappy with the standard of Nav input and started sending navs to the Tac Weapons Units (Hawk) pre OCU.
By the time I went back as an instructor (1989) There was a lot of streaming. After basic Dominies they were streamed ME or FJ. FJ students did Low level Dominies then basic JP. Then further Streamed LL or Air Defence for a dedicated advanced JP course.
I believe the ME students were also streamed for maritime or transport leading to dedicated sorties for those skills.