At the risk of upsetting a lot of folk here goes.
Ultimately a company or an organisation, in this case the RAF, is responsible and liable for the actions (good or bad) of its employees and must take responsibility.
In this case the RAF placed the responsibility for the safe conduct of the flight on the crew.
The crew accepted that responsibility.
Leaving aside for a moment the shenanigans that the RAF got up to after the accident which were nothing short of disgraceful, but also remember the old adage 'If you lie you had better be good at it' and the RAF were not.
The rules and regulations covering RAF flying operations of its aircraft had stood the test of time.
Those regulations would have covered IFR - Instrument Flight Rules and VFR - Visual Flight Rules when a crew can dispense with IFR rules and fly the aircraft under VFR when they take sole responsibility for the safe conduct of the flight. VFR rules state the required weather minima that must be observed to fly the aircraft visually. In other words the crew need to be able to see where they are going and not find a cloud with a hard centre. If an aircraft is in cloud and cannot see the ground or the direction of flight then it should not proceed on its track below the safety height.
It is just common sense that even if you think you know where you are and trust your navigation systems you just do not press on, while in cloud, below the safety height and not being able to see the ground or terrain ahead, unless the aircraft is able to use the assistance of positive radar control. There was no positive radar control available to help the crew in this accident.
In this accident the RAF tried to cover up many of the issues and absolve themselves of any responsibility.
However, had the crew followed the rules the aircraft would not have pressed on below safety height and either back tracked or climbed. Something they should have done regardless of the NI security situation at that time or the issues the aircraft was suffering from.
There are a huge number of other aspects to this accident such as the standard of training given to the crews involved in flights carrying VIPs. The culture within the RAF. The pressure on the pilots to get to their destination with their important passengers and many other facets. In my view nobody comes out of it well, but had the pilots obeyed the rules with regard to the weather they encountered then the accident would not have happened.
Obviously, had the weather been good then the accident would not have occurred.
All accidents are the result of the 'holes in a Swiss cheese lining up' and never the result of a single error. At some point a hole needs to be covered or a link in the chain broken to prevent an accident.
The rules, regulations and operational procedures are designed to prevent just such an event. They normally work until humans get involved.