Quick bit of Googling threw up a picture.
As you can see, it has a lot of family resemblance to Networker/Electrostar/Turbostar vehicles, with doors in pockets a bit like the class 376. It's 20 metres long, of course, like the chassis it was built on (a 4-CIG driving trailer).
It was built in 1997, and made a few public appearances - apparently Connex South Central were interested. The concept was a sound one - the electrical equipment and bogies of SR EMUs were bulletproof and reliable, but the bodies failed to meet modern safety and accessibility standards. They would have been quite cheap to produce, by the looks of things, but before long all three of the main ex-Southern TOCs decided on large new builds.
This vehicle survived until 2012.
I'm guessing you may be mixing this up a bit with the plans NSE had to build various other classes in the Networker family - certainly there were main-line, doors-at-the-ends vehicles planned for that, which may or may not have included compartments. Obviously those got shelved in the run-up to privatisation. Info here...
https://en.wikipedia.org/wiki/British_Rail_Classes_371,_381_and_471
THAT was built on a MK1 chassis???
That’s almost Frankensteinian!
Not that I would advocate compartments on commuter trains (my position is and always has been that a few should be available in each class on mostly open saloon IC/XC trains for those that want them), I believe that the Merseyrail Class 502 and 503 stock had a couple of First Class compartments which were either removed or declassified in 1971.
I think I remember seeing a picture of one once but can’t find it online.
They were 1/3-2/3 dual leaf sliding door stock, so it can be done (not that you would want to on a commuter service...)