As Ethel points out, the majority of people in the UK mark the box on the electoral registration form which removes their name from the edited register. Their names will only appear on the full register. This is only available in electronic form to authorised users, such as the police and credit reference agencies. (The full register remains available for anyone to inspect in printed format but you have to go to the relevant local council office or public library for that area. The printed list is produced on a 'street by street' basis, so the only way to search for someone is to look through the lists for every street in the register).
The edited register is available for anyone to buy directly from the electoral authorities but this costs 'megabucks'. Those purchasing it are either mail-shot companies (or, more usually, firms who sell services to mail-shot companies) or firms like 192.com They recoup their investment by selling the edited register on CD (as per Ethel's link) or by offering paid-for services online.
As stated above, you can't search the full register either online or on CD. Larger reference libraries used to make the CDs, containing the edited register, available but it's harder to find a library with an up-to-date disk than it used to be. (Libraries have to pay much more than the general public for the disks because they have to pay for a licence to make them available to the public. Many libraries see little point in paying a lot of money for a product which, since the introduction of the edited register in 2002, only has an incomplete version of the register. A few libraries - e.g. Camden - refuse to stock the disks because they claim, incorrectly, that to do so breaches the Data Protection Act).
Chris