The British Humanist Association web-site has reported on the results of the British Social Attitudes Survey, which is a major annual survey, the latest report covering the year 2013:
For the first time, those people saying they are not members of any religious group are in the majority in the UK, up by three percentage points from 47.7% to 50.6% in just one year. Furthermore the survey indicates that 62.4% of people never attend religious services, so some of those claiming to be members of a religion must be completely inactive.
I was never a JW and I'm not sure what this means for the WT. However it seems to me that a lot of WT teaching criticises other christian groups, saying they have got it wrong on the trinity, dates etc. - this would seem to become increasingly irrelevant given the decline in those faiths. WT may have to shift focus to attacking the non-religious, but that's a less easy target as, by definition, they have no one set of beliefs..
WT may benefit possibly in the short-term from catholics and anglicans looking for other homes as their churches empty, but ultimately it's hard to see how a decline in religiosity in the UK can be anything but bad news for the JW faith.