It also doesn't say that the survey is necessarily wrong - which is what mainstream media has leapt upon.
It says that the survey "could" have been filled in by people not from the UK, therefore the result "could" be wrong for the UK and I think it also allowed people to answer more than once.
So they're assuming that the upturn is down to people who shouldn't have filled it out, which seems possible, but unlikely to me. If nothing else, that is assuming that the people who answered the survey are more likely to answer that they are Christian than people in the UK would.
If, for example it was 1000 people (probably far more) and 20% (200 people) said they were Christian, which was a rise from 15% (150 people) last time.
I think we can expect that the majority were from the UK, so let's say 75% of people were UK.
For the UK numbers to have stayed static, that would mean 113/750 said they were Christian.
So of the other 250 (from other countries) 87, which is 35% would have had to reply they were Christian.
Now I'm sure some people would say that there are other countries where the majority would say they are Christian - but why should people from those countries have been more likely to fill out a YouGov survey than people from countries where Christianity is a vast minority?
Equally well, is a Christian more likely to fill out the survey twice than a non-Christian? I doubt it.
And there are other signs in Bible sales, testimony and church figures that backs the revival. I'll accept it may not have been as large an increase as the YouGov survey might have initially suggested, but I suspect strongly it is there and the negative headlines put out there are from people not wanting to think there could be a turning back to God.
Edited to add that most denominations do some version of numbers checking - church I work for is associated with several denominations and all require an end of year set of figures for working out parish share as well as their own statistics. I have proposed that the church goes Baptist as the easiest parochial figure document, however they're still after average numbers etc.