To me, it's whatever infrastructure is felt necessary to ensure safe operation. It would probably need an independent review as I doubt if everybody would accept anything done by a TOC/NR - or any Union.
In practice: GSM radio coverage, sufficient lighting on the platforms and functioning DO train-mounted cameras are the infrastructure changes needed nowadays. GSM radio was installed nationwide a couple of years ago, where previously only the South East and some pockets of Edinburgh and Glasgow for the most part had a radio system fit for DOO-Passenger, and therefore those were the only places British Rail introduced it. DOO non passenger trains already run across the whole country’s infrastructure. Authority to run DOO passenger trains would be given by Network Rail under standard processes. ASLEF or the RMT would not be involved beyond checking that the laid down regulations had been followed.
In terms of the latest standard in-cab DOO systems installed on brand new trains, it would be expected that they should pass any ‘fit for purpose’ test quite straightforwardly.