CCTV on the Underground trains that has it fitted allows the Train Operator to view the train as it leaves the platform. Of course there are a few differences such as some Network Rail stations not having a signal at the end of the platform which may slightly increase the risk of a Spad, but I myself have had no issues with monitoring the PTI at the same time as observing the road ahead.
It's certainly possible, but it's definitely not ideal by any means. Certainly on LUL looking at the monitors has been cited as a causal factor in very many SPADs on the lines fitted with in-cab monitors.
I would suggest that this risk is tolerated thanks to the SPAD risk on LUL being mitigated by the trainstop/tripcock system, or other forms of ATP, and the (virtual) guarantee that the train would stop within a designated overlap.
On the mainline there is no guarantee that any form of protection will be provided (TPWS not universal), plus on the mainline the overlap is provided to cater for driver misjudgement rather than disregard.
I would concur with others that on the mainline I would prefer for the driver's attention to be focussed on the road ahead.
Having said all of this, the most important safety-critical part of the process is the safety check between closing the doors and starting to move the train. In this respect I would view in-cab monitors as perfectly adequate, but they should definitely switch off the moment the train starts to move. On LUL it's fair to say you cannot *rely* on the driver picking up an unsafe incident in the monitors once the train starts to move, as drivers are taught to balance their time between checking the monitor and observing the road ahead, the simple reality is it's impossible to reliably carry out both tasks at the same time - and this has been officially acknowledged in human-factors studies.
It's also worth adding that the in-cab monitors on LUL are not fantastically reliable, due to the realities of the technology available. LUL mitigates against this risk by having procedures that rely on station staff being available as a fall-back at platforms where all doors cannot safety be seen from the cab. On the mainline many stations would be unstaffed or the staff present might not be safety-critical.