Radio 4’s Sarah Montague got flown south along the Derwent run in a GR4 a couple of months ago and the
Watching it from about 12:30 you can see that the valley is much too twisty and narrow for a fast jet to stay within. Also you can see that the cloud base directly over the dam is less likely to be a problem than the cloud base over the high moorland to the north. Fast jet low flying rules require a minimum of 500 feet vertical clearance from cloud, and with the minimum flying height being 250 feet this means a cloud base of 750 feet above ground is the absolute minimum needed for legality. These rules are in place to allow ‘see and avoid’ against light aircraft and helicopters, which can quite legally potter around ‘clear of cloud in sight of surface’ in much worse conditions. The only place in the UK where the IMC capability of Tornado can legally be used for sustained low flying is the segregated airspace R610 over the Highlands.
Flypasts usually have to be higher than 250 feet as well; this would have compounded the weather issue and maybe required a cloud base as high as 1000 feet above ground. To be honest, given the descriptions given, it sounds as if things were being pushed a bit. The pressure of an audience is the one thing that worried me most as a flying supervisor, much more than anything on operations; people can take surprising and unwarranted risks in the heat of the moment, as the litany of display accidents in recent years amply demonstrates.