This the standard practice on on trains with guards on Southern, South Eastern, Virgin, Overground and Cross Country, and almost universal in the erst of Europe, including Eurostar and TGV. In fact, the guards control panel on all the relevant trains does not have a door release button - just door close and bell signal buttons. Southern now even operate the 442s in the same manner.
So why on earth can't SWT do it? I'm fed up with the several seconds wait at each station while the guard has to check that the platform hasn't been abducted by aliens before releasing the doors.
Chiltern 165 and 168 services work like that too.
Chiltern is a funny old TOC when it comes to DOO work. Everything south of Banbury (except for the Bubble car, which makes a handful of trips a week) is DOO, and the services which extend beyond Banbury are conventionally manned with a driver and guard when they are north of Banbury, which includes dispatching from the down platforms at Banbury station.
Chiltern are now introducing loco hauled trains which need a guard on all the way from Birmingham to London, and they are going to increase the number of these loco hauled trains quite dramatically over the next couple of years until pretty much all of the Moor Street - Marylebone "fast" services are operated by a class 67 & mark 3 coaches.
The reason is that they obviously need to cascade the 168s onto the Oxford branch when that opens, you cant just run a 100mph half hourly service to a new (and massively important) destination on your route without it having a pretty serious impact on your fleet and how that fleet runs. Oxford services will be DOO. 168s and 165s are built to make DOO work easy. 67s are not.
This leaves me thinking about the current traction we have on the railway. Talking purely hypothetically for a moment, if somebody made a magic wand and the entire network had to go DOO next year, which units would easily allow for that to happen (I am guessing most of the networker/turbostar/electrostar family of trains) and which ones would it be a lot more difficult to modify?