(2-part post):
There are two versions of the electoral roll. The full version (which contains the name and address of every adult who has complied with their legal obligation to register to vote) is only available in electronic form (whether on CD or online) to authorised users, such as the police, security services and credit reference agencies. All searches by these users must be recorded and are subject to audit. (Anyone who uses the database for unauthorised purposes faces a possible prison sentence).
The only way that 'Joe Public' can access the full electoral roll is to study the printed version at the local council office for the relevant area. This is listed in street order, not alphabetical street order. So, to look for 'Fred Smith' you have to look through every individual street listing, rather than just looking under 'S'.
The other version of the electoral roll is the 'edited' one. This is available to anyone who's prepared to pay a large amount of money to buy it. Typical users are junk mailing organisations and, of course, firms like 192.com. When people comply with their legal obligation to register to vote, they're invited to mark the box which removes their name from the edited roll. Over half of voters now mark this box (to reduce junk mail?), so the version of the electoral roll on 192.com now only contains a minority of the adult population.