Hi Edgington
I believe the original reason for change over to 'traditional' navigation for the missed approach segment in the UK, was that some RNAV equipment of the time would occasionally turn in the wrong direction even though coded correctly. This is the reason you will see more recent RNP approaches being designed with a couple of waypoints in the missed approach to ensure a turn in the correct direction. I assume that this has been taken into account by the CAA prior to publishing CAP 1926.
Cheers
TeeS