The answer is racism. The modern concept of racism has allowed slavery to be re-introduced into modern Britain.
Thanks to half a century of ignorant and foolish propaganda on the subject from the United States, most people are under the historically utterly-false impression that slavery is about racism - specifically the racism of Western Europeans towards those of different skin colour.
This is absolutely untrue: it is just about as untrue as it is possible to be - yet that is what people seem to believe because that is what they are fed by the mass media.
However, modern slavery is typically either slavery of Europeans (in the form of sexual slavery/ human-trafficking), or it is the supposed ethnic 'victims' of racism who are themselves the slave-owners.
It is groups who are themselves depicted as victims of racism that have actually brought slavery back into the UK. In a sense this is quite well known, since the media has been reporting a steady stream of cases over the past decade (one from within a mile of where I live).
Because these must represent the tip of an iceberg, there are probably some thousands of slaves in Britain now, and even more (especially women, and also probably mentally handicapped) whose lives are more like serfs - not 'bought and sold' but permanently-tied to a particular family.
But it is all hidden in plain site, people are 'scandalized' for a few hours but don't really know what to make of it - it doesn't fit their world view, it doesn't fit their moral map: so they soon forget.
So it seems nobody is bothered about slavery to the extent of actually stopping it; despite that slavery was successfully abolished in Britain hundreds of years ago - apparently the modern surveillance state cannot achieve what the late medieval state managed...
Of course the real reason modern slavery is tolerated in Britain is not hard to understand. Modern slavery is invisible because people falsely believe slavery is about 'racism' by which they mean whites enslaving blacks; and when in real life it isn't, then the fact that anti-racism is the primary moral value of modernity means that to prevent or abolish modern slavery would itself be regarded as an act of racism.
Anyone who did anything decisive to eliminate slavery and serfdom from modern Britain would themselves be denigrated as racist.
Better to say nothing, do nothing, see nothing...
So the reintroduction of slavery has been - tacitly - accepted by the British ruling elite, so long as it is not what they call racist - actually-existing slavery is seen as just one aspect of the vibrant multiculturalism which our elites affect to desire - regrettable indeed, but not regretted to the point of actually stopping it...
I regard modern British slavery and serfdom as perhaps the most decisive refutation of Leftism, of political correctness. If slavery is happening in the modern state, then it seems that anything is possible - and it is, and it is.
THIS is the world built by political correctness, by Leftism.
No wonder the elites prefer not to talk about it: to pretend it hasn't happened; and do nothing.