Only opening 4 out of 12 coaches is perfectly possible with standard dwell times, provided it is only at appropriate stations. SWR/SWT have managed it for years at smaller stations, I have a feeling there may be some Southeastern stations which are like it too,Yes, it wouldn't be normal practice as you'd need really long dwells. As this was unexpected (may well be the only time it has ever happened) it stayed there for a while.
The way UDS works is that the guard uses the front cab at the front of the last unit that *isn't* to be released. So in a 12-car if you use the third cab back you can lock out all but the front 4.