Interesting, I hadn't appreciated that (who would if they don't work in the indutry?). Sounds a bit like building your own PC, it is not that difficult but you have to be careful with your component selection to ensure everything is compatible.
I don't work in the industry, but it's stuff I've got to know over the years!
BR were actually well on the way with this - all of the second-generation DMUs use BSI couplers and nearly every combination can multi-work quite happily between classes. The second-generation EMUs used Tightlock couplers, and could similarly work together.
Then when it was all split up into separate operators there was no rule stating that compatibility was required (which there should have been). 170s and 172s have BSI couplers so can work with second-generation BR classes, but apart from that each manufacturer did their own thing (they all used Dellner couplers, but with no consistency as to whether the control box was above or below, and no standard control protocol. A standard height did evolve, but some classes such as 22x and 390 are different. The control protocol sometimes changed between generations from the same manufacturer so even if the heights are the same and the units can physically couple, they often cannot work in multi so any coupling of them is for rescue purposs only - I believe this is the case with 222s, for example, which cannot work with 220s or 221s.
With locos BR weren't particularly consistent - the first generation of diesels mostly used blue star (with a few exceptions), but after that a number were in-class only. with electrics they eventualy standardised on TDM, as fitted from new to 89/90/91, and retro-fitted to 86 and 87. With privatisation it worked better, largely because of the dominance of GM in new locos at the time, hence the US standard AAR multi-working system is found on classes 59/66/67 and later 70 (and fitted to all flavours of 73/9 during rebuild). The 68s and 88s use a European system and can work with each other but nothing else in the UK. The 'Chiltern' 68s have been fitted with what I understand is a cut-down version of AAR additionally, which allows them to be controlled from an AAR-fitted DVT, but not to work in multi with other AAR locos.