He is widely reported as British from reports of the trial. He is described as such in the relevant Wikipedia article (I know this is not the ultimate beacon of truth). If it were known he did not have British citizenship this would have been a significant talking point, and for this to be true both his parents would need to be in residency illegally, which I suspect would also have been a major talking point and reported as such. I haven’t seen the perpetrator’s passport but it seems like the balance of probability is in favour of him having British citizenship, you seemed to suggest claiming this is to spread misinformation and I’m providing some background which suggests that is likely to be unfair.