There is no problem with it continuing in service until the end of the day. If something has failed in the DVT cab requiring the train to be reversed at Newcastle so that it can continue to London and then a second loco attached so that the train can be worked as a "top and tail" formation, there is no reason to take it out of service. The failed DVT is "boxed in" or "buried" in the formation by the second loco. Job done. Problem solved.
As for turning the train at Newcastle, this is the difference between allowing a faulty train to continue on it's way or being cancelled. If, for example, the windscreen on the DVT had been broken, the driver can simply change ends, leave via High Level Bridge and the DVT is now at the rear of the formation.
A failure does not automatically mean that a train has to be taken out of service, nor does it mean that a faulty DVT cannot be used for an ECS move to a maintenance depot (even if it would be required to run at a reduced speed).
O L Leigh