Someone mentioned a radar altimeter.
On "private" instrument approaches, are the CAA-authorised operators required to use a radalt? That would seem most unusual. Even Cat 2 and Cat 3 approaches do not require the pilot to look at the radalt. The autopilot sure uses a radar altimeter to do the last stage of an autoland approach, but on any manual approach the pilot still uses the standard altimeter for the DA decision.
A radalt would be extremely handy for DIY approaches though.