I bet that in ATCO's manuals the priority order in favour of commercial traffic, is written somewhere, as it should be!
In fact, as other posters have commented, it states quite the opposite. All airways traffic following a filed flight plan have the same priority as each other. It is aircraft in an emergency, Royal flights/Heads of State flights etc that have the greater priority.
That saying, a light, slow (compared to turboprops/jets) aircraft often causes inconvenience to the controller and so can be expected to be vectored around the sky to make way for the faster aircraft. It very often depends on the time of day and current traffic situation.
I agree with <
anotherthing> in saying that if you get airborne knowing you might not get your requested flight level or exact route, then you should have no problem with your flight.