"Is there a prescribed route that learners have to use or is it instructors just being awkward?"
Driving instructors use as near as possible the routes used for tests, this gives the learner the chance to know any pitfalls for a particular manouvre when they eventually take the test. Learnerdrivers need to experience as many possibilities of road conditions possible therfore rushour traffic is one of them. Far from destroying confidence it can actually bolster confidence.
I take that you drive and so were a learner driver once. Give them a break and stop whinging. Go out a little earlier!