flip tb was considered eradicated in countries like the UK. in addition, the vaccine (like most) is only effective to a particular extent, against particular strains.
unfortunately it was not eradicated, particularly in low and middle income countries, and there are many millions of people who, unwittingly, are passive carriers of the disease. most of the time a diagosis is only made when the person develops active tb
with the increase in global travel and immigration, more people picked up the disease on their travels or people who had the disease (either in its active or passive form) moved to live in countries like the UK. hence incidence of the disease increased again.
one of the risk factors for spread of the disease is people living in cramped conditions, so when you think of people maybe living in a shared house or with family in a comparatively small space, then the spread of the disease becomes much easier. prison populations also tend to be prone to TB for the same reason.
the increase in tb in London, with its large immigrant community and substantial tourist population, is why the local authorities have started to provide the jab again, either to "at risk" children or as a matter of course.