The link you quote is about religiosity in the US - do you have a similar link to anything specific to the UK situation?
There was also a recent survey of the breakdown of catholics in England and Wales, which showed a similar finding, but with knobs on.
So just 0.9% of current catholics were brought up in a non religious manner. 92.3% of current catholics were brought up catholic, although a catholic upbringing isn't very 'sticky' in terms of retention as only 55% of people brought up as catholics remain catholics as an adult.
The study also made comparisons with the other major christian denominations in the Uk, with similar findings, so 2% of current anglicans were brought up non religious, same for baptists and 0.8% of methodists were brought up non religious.
There is also a tiny proportion of current christians in the UK (0.1-0.4%) brought up non christian religion.
So except for a couple of % current chrstians weer brought up in their current denomination or were brought up in a different christian denomination. So the only meaningful 'conversion' to christianity in numerical terms is from one christian denomination to another.