I have in the past installed using only the containment (conduit and trunking) as CPC. However, after a long discussion and analysis with my colleagues, based on real-world findings over multiple installations, we agreed that the number of examples of suspected and measured high-resistance connections in the containment was too high to ignore completely, and since then made it a principle to specify a copper conductor throughout. This was not a matter of achieving compliance, we were happy that the containment was sufficiently reliable to comply and to be safe. It was our own specification that we work to, which exceeds minimum requirements in most aspects. We will accept and re-use existing containment CPC where it is sound.
There is a certain amount of common-sense involved. A stiff backbone of 200x50 trunking along the wall with short 25mm galv drops, is not going to put any one point in jeopardy of losing its CPC. A long run of a special-purpose circuit that snakes around the building in a solitary conduit that's often inaccessible, is an order of magnitude more likely to have one or more of its many dozens of joints loose or subject to corrosion. Here it makes statistical sense to run the copper CPC as ductering and tracing one possible marginally high connection is not going to happen in the future. The actual resistance of the conduit when good will probably beat the copper hands down, but the copper continuity is predictable for the life of the installation in a way that 97 screwed joints, one of which is subject to rampant condensation near an unseen gap in the eaves, might not be.