Pontrilas Army Training Area
The area, north of the village of Pontrilas in Herefordshire, close to the Welsh border, seems to be a former munitions depot, judging by the (dismantled) train line system, just west of the training area's main gate, near the village of Ewyas Harold.
The old disused munitions train line system once connected into the nearby main line at a branch junction in the village of Pontrilas itself, a little further south.
The plane is rumoured to be used for counter-terrorism training. Rather than a real jet, it seems likely that the aircraft is in fact merely a "mock-up", similar to those used on film productions.