It's not something I'll get too concerned about, but I'm with spin on this. Even if you ignore the regs (what a nice thought!) a main switch would be the single switch which controls the largest amount of circuits after the head/meter. So it has to be the first isolation point after the head/meter.
When you consider that there can be up to 3 metres of cable between the isolator and the CU which the CU switch does not cover then the switch in the CU can only isolate the installation partially even if it is the largest part.
That said, from what I can see of paperwork relating to installations 90% is worthless or missing. That which does exist is rarely used or seen by anyone other than the spark who wrote it, and maybe another spark working on the system at a later date. So you could probably note the transformer up the road as the main switch and no one would say anything.