snaga
The commander does not lose his direct responsibility, however, if instructed by the UK Air Defence Authority (either directly or by ATC) to take a course of action and the commander decides not to comply, then his responsibility will also be stretched to include the resultant actions taken by the authorities.
As SixDelta suggests, the urge to not comply with an armed fighter sitting off your wing either displays huge cojones or a complete misunderstanding of the situation you are actually in.
It does however seem that the handling by the authorities once on the ground needs a big shake up.
As a Devils Advocate, for aircraft travelling to an adjacent country then they could perhaps even consider escorting such aircraft clear of UK airspace (remaining clear of densely populated areas) and let some other authority deal with it in the future ??