I have often thought this myself. BR was standardising on BSI couplings in the latter part of the 1980s for second generation DMUs
And Tightlocks for all EMUs; except for those just for the old Southern Region (442, 455, and 456), they also had integral electric and pneumatic connections (I've heard this add-on described as
"the Preston Box" in the past).
BR tried Scharfenberg-made couplers on the PEP prototype EMUs; I was told by a Southern Region traction engineer many years ago that it proved extremely difficult to couple/uncouple the PEPs on the many curved platforms found on the old Southern Railway lines, and as Buckeyes had been proved to work ok, the Tightlock (very similar to a Buckeye) was adopted as the BR EMU standard. I don't know why the BSI was adopted for DMUs.
Adtranz used the BR standard couplers (and coupler heights) for Turbostars and Electrostars; the change to the Dellner head - initially on 377s - was a customer requirement, but my understanding is that it is possible to change back DMUs/EMUs fitted with Dellner heads to BSIs or Tightlocks if required.
In theory, northern's class 319, 321, 322, 323, and 769 should be able to couple - not sure about operating in multiple though - and all of it's ex BR DMUs plus the Turbostars should be able to work in multiple.