its not really the coupler that is the standard, its the electrical and electronic connections netween the trains that makes the compatibility.
Take for example SR 27 way cables. The actual coupling between units is standard buckeye. However, while most of the 400 series emus were compatible with each other and 73/1 and 33/1 they werent compatible with 73/0 which were wired to be compatible with the thumpers which used a different wiring setup although the plugs were the same! 455 use the same coupling principle, i.e MU jumpers at platform height avoiding shunters having to go on the track to connect and therefore avoid being near the juice rail, but the coupler is different, its a tightlock. (i think) now, they can theoretically couple to a 319 and electrically they should be the same, but 319s dont use platform height jumpers!
pendolino and voyagers are coupler compatible, one can rescue the other, however because the electrical systems are different along with the electronics, the train TMS etc they wont work in multiple, and this is a crucial why trains arent compatible. TMSs are different. meridians and voyagers use the same TMS, but because Virgin and Arriva have since specified their own unique differences to train running, the TMS will be programmed different to EMT/EMRs TMS thus rendering any software compatibility useless. Same with the desiro family. while the 350, 360, 450 and even 185 should theoretically couple and work in multiple, the differences in software on the TMS will mean the trains wont talk to each other. I believe such differences mean 350/2 cannot work with 350/4 although the /3 and /4 do and have worked together, Southern have the same issue within their fleets as there are issues with the 375, 377 and 379 fleets not talking to each other due to different TMS versions. So I understand things anyway, happy to be corrected if wrong.....
Its like going back to the humble class 47, the most non-standard of the standard type 4 locomotives in BRs fleet!