There's no doubt whatsoever that a full possession would be considerably more expensive - and over the top - than red zone working with a lookout! At least one member of the gang would almost certainly hold lookout competency - it's quite common - whereas a possession would require at least a person holding PICOP competency and either detonator protection or possession of the token (so would probably need to travel to a different location in a different vehicle), an ES (is that still necessary with just a single worksite?), possession planning and a need to publish the arrangements in advance. Not at all simple!
As others have said, no-one should ever rely on the published timetable or their own knowledge of train movements for their protection. We have a branch line that normally sees only a single train each way, Monday to Friday only and never ever at weekends - apart from last Saturday when there was a problem on Friday and they left the train (or at least part of it) there overnight. I came very close, on another occasion, to putting a train onto another branch, which hasn't seen any regular traffic in years, just to get it off the main line during disruption to allow other traffic to pass.
This link, to a description of the Staplehurst accident in 1865 is also a good illustration of where that sort of assumption can end up!