I imagine Driver only operated (DOO) on IEP will only happen in the areas DOO is currently allowed. So Cotswolds east of Oxford, and then everything else West of Didcot Parkway/Bedwyn.
The Train Manager (TM) could still be on board until London Paddington (PAD), but if running late, the TM can be dropped off at Reading (RDG), take their Person Needs Break (PNB), then take up their original working from Reading back West. This currently happens when the brown stuff hits the spinny thing with drivers, but with London Trent Valley (LTV) HST trained drivers covering to PAD and back to RDG.