I have heard that the examiners use set test routes, is this true?

Yes it is. The examiners do have a pre-defined set of test routes which they follow.

Or at least they do as far as they can. If there are local difficulties then they deviate from the routes.

A good instructor will teach you to drive. Not merely to follow a set of test routes in a drone like manner. Any test centre will have loads of test routes. I know instructors who follow test routes slavishly. I know young people who passed their tests with such instructors and when they leave their known areas the nerves become apparent. Because they do not know where they are! Pathetic.

Bear in mind that the driving test itself is only thirty five to forty minutes long. If there are twenty plus test routes to an area then the test routes will cover all of the roads and junctions that you can reach from the test centre in that time limit. Add to that the fact that the route may be deviated from at any time and for any reason that the examiner decides upon and they are pretty meaningless. The only two reasons that they exist are:- so that when the examiners leave the test centre they are all going to different places, and so that every candidate has an equal test in terms of difficulty. Both great reasons.