The other version of the electoral roll is the 'edited' one. This may be sold (for a very large amount of money) to anyone who wants a copy, such as mailing organisations or firms like 192.com. However, when people register for the electoral roll, they're invited to mark the box which excludes their details from the edited roll. The majority of people probably choose to do this (in order to cut down on junk mail), so the edited roll probably includes well under a half of all the adults in the country.
So, however you view the electoral roll in electronic form, there's no certainty that you'll find the person you're looking for. Also, because firms like 192.com pay extremely large amounts of money to purchase the rights to use the edited roll, you'll never find a genuinely free search service online. However, the firm behind 192.com also sell the edited roll on CD-Rom. Many public libraries purchase these CDs and you can use them, free of charge, to search the full national edited roll in the library.
Head down to your local library and you might find what you want. However, if you're unfortunate enough to live in Camden, none of the libraries will have the CDs because they (wrongly) state that allowing you to use them breaches the Data Protection Act.
Chris