There are two kinds of things in life
1) mandatory
2) all others
If something is in category 2) then why try to get a logbook entry? One may as well get one's logbook signed for attenting a CAA "safety" seminar
(and yes people do actually do that; they all queue up at the end for that magic rubber stamp which - coming from the ultimate arbiter of airmanship - guarantees immortality)
Today, you can buy a plane with a big LCD which integrates a GNS530 and a load of engine instruments, and perhaps even an autopilot.
5 years ago, you could have bought the same plane without a big LCD, with (going off somewhere for a copy/paste...)
KLN94 BRNAV IFR certified GPS
KMD550 multifunction display (GPS moving map + stormscope)
WX500 stormscope displaying on KMD550
KCS55 slaved HSI
KI229 dual-needle slaved RMI (ADF+VOR)
2nd altimeter
2nd electric horizon
EDM700 6 cylinder CHT & EGT monitor with data logging
KN63 DME
KR87 ADF
GTX330 Mode S Transponder
2x KX155A COM/VOR/ILS (dual independent ILS)
PMA7000 intercom with markers & music input
Shadin fuel computer, calibrated, linked to KLN94B GPS
KFC225 autopilot with flight director, ALT and VS preselects, tracks VOR/GPS/LOC/ILS and is authorised for an ILS down to 200ft AGL
and what training do you legally need for that? Absolutely none. It would be up to the pilot to read the manuals and to dig out somebody (most likely an airline pilot; hopeless looking for instructors who know) who can work it out and explain it.
Paradoxically, one needs legal training for a CS prop, which hardly needs any. One doesn't need any legal training for that mysterious red knob, but that actually needs quite a lot of training to be used correctly.
One could argue that all significant avionics should need mandatory training, and I would agree, but where the hell would the instructors come from? They would have to be imported from the USA! To train this stuff, you need to be somebody who actually uses it, and nearly all UK instructors never fly anywhere.