Does sex trafficking happen? Yes. Is it on the scale you think? Very probably not.
I've been pointing out the dodgy basis of the statistics for a while. My particular favourite flaw, mentioned in the article, is the inclusion of women coming to the UK to marry in the 'trafficking' estimates. 'I wouldn't, therefore you mustn't, therefore anyone who is must be a criminal.'
There's one other statistic on the sex industry that I know is crap: I made it up, and made it clear to the person who asked that it was made up. They still published it and every so often, I see it repeated. It makes a really easy test to see who's talking rubbish.
Let me see if I can find a good article on one recent convicted 'trafficker' whose crime was to book someone a taxi...