From BS7671's point of view, RCDs can be required for fault protection, impact protection, or additional protection.
It can often be a reasonable limitation of the EICR (front page) that they aren't going to excavate every wall to check the depth of buried cables, meaning Impact Protection would have to be very obviously deficient to code it.
It's quite likely that for the storage heaters the installation meets the current regs for fault protection and additional protection (section 411.3)
So I do actually find it completely plausible that a sparks who understands the regs might not code it at all.