First, the survey says "regularly", defined as "usually attend church on Sunday", not "every Sunday".
So what? In practice there is very little difference between the two things. Do you think that people who do not attend Church
regularly have no right to call themselves Christians?
What is the Church though? Personally I believe that the Churches people go to on Sundays and on other holy days are creations of humans, not of God, and that
the Church is something altogether different.
You are missing the point here; religious culture in Britain differs from religious culture in America is one fundamental way, namely that religion here is a very personal thing, rather than something more collective (with the interesting exception of singing hymns and the like; something we've always liked).
As such, Church attendence in Britain has
always been significantly lower than in the United States; even in the 19th century (when the social pressure of Church going was at it's greatest) only a minority of people could be considered to have been regular Church-goers.
It's significant that the more radical Protestant groups in Britain (going all the way back to the Lollards)
always placed great emphasis on Bible reading, individual religious observation and religious observation within groups smaller than conventional Churches.