Afraid I am going to get a splinter in my backside, because I am sitting on the fence on this issue! Had a very similar discussion only a day or so ago. Bottom line is, yes, it is acceptable to rely on a properly installed and connected steel trunking system for all cpc's of all circuits. All earth links must be properly fitted, cpc's run to earth terminal of every accessory (socket outlets, switch plates, luminaires etc),and it must be tested thoroughly and achieve results that conform to BS 7671:2008 (2011). The cpc's must be terminated onto the trunking and bolted through effectively etc etc. But I am also aware that many installers prefer to instal separate cpc's, for the added security that they feel a separate cpc offers, should, for any reason the containment system fail. There is of course extra cost but that is their choice. In reality it matters not, the requirements of the Regs. are fulfilled and everyone is happy. I have installed in both ways, mainly the latter in recent years as architects/Q.S's etc have specified it, for reasons of their own, hence I feel the custom has become widely adopted (but NOT necessary). And if it means extra cabling and bigger trunking (and conduit) etc, well good luck with the price if you are tendering against someone who is not installing additional cpc's.