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.

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, and certainly less than the problem caused by this failure. Furthermore, I would guess that the mitigation would be just to manually direct the flight according to the accepted flight plan, as it was a completely valid one.

One of the many problems here is that they could not identify the problem-triggering 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.