I'm not sure, the BBFC used to certify games in the UK before we got the PEGI ratings, and it was a bit weird. Some things would be banned outright and get no certification (Manhunt 2 for example) and a lot of games that you'd think would be rated 18 actually got a 15. I remember getting the original C&C free in a bundle of games with our first Windows computer when I was a kid and that has a cutscene where Kane shoots Seth point blank in the head so probably that, but then it is the BBFC so I may be misremembering. If not then the original Half-Life, that was pretty gory for the time and I think that was an 18. It's strange how we absolutely did have the video game panic in the '90s and '00s that the US did, but the BBFC didn't crack down on it anywhere near as strictly as it did the "video nasties" of the '80s.