There's probably 2-3 serial killers currently active in the UK at any one time. They just haven't been caught yet.
A lot of killers choose victims who nobody cares about, so... homeless people, asylum seekers, prostitutes, drug users and young people who've been involved in the care system are 'easy' targets, as nobody notices they're not 'there' anymore. And, even if somebody does report somebody from such a 'group' as missing, police do not generally waste too much time trying to locate them.
According to 'The Independent' (2017), 270,000 people go missing in the UK each year. According to 'Missing People' (2016) that figure is 180,000.
A good proportion of those people are either 'wanted' people, such as criminals, or 'illegal immigrants/asylum seekers without status': and they simply 'disappear' into kebab shops, sweatshops, cash-in-hand jobs, use a new name, stay under the radar.
But what happens to the rest of them?
Nobody knows. And, as most of them are 'just' drifters and junkies, nobody really cares.