According to government research for the DCSF, 90% of pupils have computer and internet access at home. Even amongst primary school children, it was 88%.
The research was part of the “Parental Involvement in Children's Education” report, published in 2007, and page 25 gives the statistics for Internet access.
There’s an interesting breakdown by social grade in Table 2.3, but even amongst the most disadvantaged groups, the access is still over two-thirds:
% with internet
access at home
I came across this through one of my RSS feeds on TheyWorkForYou. I use it to watch for news and announcements in parliament about education, and often it comes up with interesting nuggets. The RSS feed I use (this is the link) provides all of the answers Jim Knight provides to MP’s written questions.