I can’t speak for what Great Britain or some other government does, or what you mean by ‘target practice’. In the United States, for aerial gunnery practice, we employ a number of unmanned air vehicles as targets, most notably, the QF-16, which is a retired F-16 fighter converted for this purpose. I have never heard of light general aviation aircraft being used as targets for aerial gunnery exercises.
General aviation aircraft are sometimes used to simulate threats where the military assets interact with them, but do not fire upon them. One example are joint exercises with the civil air patrol. CAP will use Cessna 172 and 182 aircraft to intentionally violate practice TFR areas, allowing for military fighters to fly simulated intercepts. And it comes in handy, as NORAD reports hundreds of TFR violations every year; mostly people with drones, but there are occasions where some ding dong doesn’t read the NOTAMs and flies his Piper Cherokee into a presidential TFR or similar event. They are generally intercepted and handed over to law-enforcement on the ground.
It is possible as well, that military aircraft may track civilian aircraft using fire control radars in exercises for practice, similar to what was described above. I do know the military has also used civilian ground structures as practice ‘targets’ in simulated air strikes. I remember reading stories from Colonel Tony Tolin, who used to command the 37th TFW (F-117As) out of Tonopah, NV. He said that, during the 1980s, their aircraft would fly specific missions at night, targeting specific buildings or structures discussed in preflight briefs as a simulated strike mission in preparation for war. I remember one case where Mr. Tolin stated that their target is a specific cabin with a dock on the north shore of Lake Tahoe. The only thing missing was that the jets never dropped the bombs (nor did they carry any on these flights), and for additional safety, never lased civilian buildings as well.