@TheOnlyCherryOnMyTree I think there are multiple factors contributing to people being exposed to, buying in to, and even not recognising how dehumanisation is a tactic used repeatedly against groups defined as 'other'.
One potentially significant factor in the UK and similar countries is the legacy of colonialism - there is a history of centuries' worth of going to places where the people were 'other', 'civilising' those places (through questionable means) and sustaining rule in those places, often through force. The starting point had to include a lack of parity of esteem for the people and culture in those places. While much of colonialism has faded, there are lasting influences on how people perceive difference and inequality.
There is also the effectiveness of media influences and normalisation of it in society. Having a group that is 'other' helps maintain people's sense of superiority, which some people like to have, and sense of unity/belonging - this is a phenomenon we see throughout society, which is exacerbated in a society which is increasingly focused on individual status and increasingly disconnected. The media and politicians have fed into this, such that people are exposed to it on a daily basis, sometimes explicitly and sometimes more insidiously.
Another factor is that people are quite desensitised to violent and traumatic images anyway, now. Many people can look at the images of traumatic events and not feel the same instinctive horror that people might have years ago, which is then compounded by not being able to take the step of "that could be me/my loved ones". So, no matter how horrible the image, for many people the reactions are: "well, they deserve it", "it's a shame but...", or "I don't want to see that so I'll look away".