For the most part (although there are important exceptions), IFR flights are always in radar contact with a controller. The flight plan is tool allows ATC and the plane to agree a route so that they don't have to be constantly communicating. ATC 'clears' a plane to continue on the route to a given limit, and expects the plane to continue on the plan until that limit unless they give any future instructions.

In this regard UK ATC can choose to do anything they like with a plane when it comes under their control - if they don't consider the flight plan to be valid or safe they can just instruct the plane to hold/divert/land etc.

I'm not sure the NATS system that failed has the ability to reject a given flight plan back upstream.

Mostly yes; however, there are large parts of the Atlantic and Pacific where that isn't true (radar contact). I know the Atlantic routes are frequently full of plans that left the US and Canada heading to the UK.

I have no idea what percent of the volume into the UK comes from outside radar control; if they asked a flight to divert, that may open multiple other cans of worms.

> If they asked a flight to divert, that may open multiple other cans of worms.

Any ATC system has to be resilient enough to handle a diversion on account of things like bad weather, mechanical failure or a medical emergency. In fact, I would think the diversion of one aircraft would be less of a problem than those caused by bad weather. Furthermore, I would guess that the mitigation would be just to manually direct the flight according to the accepted flight plan.

One of the many problems here is that they could not identify the problematic flight plan for hours, and only with the assistance of the vendor's engineers. Another is that the system had immediately foreclosed on that option anyway, by shutting down.