Church attendance is down to about 5% of the population, and that's on a fairly loose definition of "attendance" (source: yougov). Weekly attendance is about 1m per week, though I forget the source of that. Even if you were to double either claim for people not responding as you say, they're both markedly less than any outlandish census claim of religious belief.
Christianity is at a very low ebb, whatever you think.
Being more picky about the figures than anything - I know many of the more sect like brethren groups tend to go somewhat under the radar when it comes to official figures for various reasons - anyone working in any kind of reasonably sized business probably works with atleast 1-2 who won't outwardly admit to being part of such a group or on any kind of survey but attend like a gospel hall regularly.