Hence me saying 3 couplers not 4.
'Dellner' is not strictly a coupler type, they manufacture the same thing as Scharfenberg under licence. When you include suffix numbers 10/12/330 they are still variants of the same coupler and are intended to be mechanically compatible if built to drawing. RSSB's standards documents on couplers group them all together as coupler type M007. (They highlight that the 390 is fitted at a 'non-standard height'.)
If you discount the 458's obsolescent AAR, (which is being removed under the current work), post privatisation EMUs are effectively standardised on Dellner/Scharfenberg, I think because it is also the specified coupling under TSIs.
There is probably no appetite for refitting all the earlier 'Tightlock' fitted EMUs, as in general they'll never need to multiple with a modern unit.
As always though, the electrical connection boxes are a complete nightmare of individual design, as are onboard electronic systems. Mechanical coupling for rescue purposes will probably remain the only possibility even if all couplings can latch together in the medium term.
It also seems to me that the BSI has been allowed to become the de-facto small [1] DMU standard because until recent times the country has been assumed to have huge areas that are mainly DMU and others that are mainly EMU operated, and there are only relatively minor overlaps; so no-one has really seen DMU/EMU rescue compatibility as important. (The 171 is the only evidence of this.) But you can think that a TOC such as Northern or FGW wouldn't have seen much to worry about.
[1] ie not 220/221/222
Of course the expansion of electrification will probably put a different emphasis on this.
