When the register is published, it comes in two forms. "Anyone can look at the full version of the register under supervision but it can only be passed on for certain purposes. These are electoral purposes, law enforcement and by credit reference agencies. Use by credit reference agencies is only to check your name and address if you are applying for credit and to help stop money laundering." A partial version can be bought by anyone and used for any purpose - but "you can choose not to be on it by ticking the opt out box on registration form."
In the UK you register as an elector - but do not, as in the US, have the option of registering as a voter for a particular party. In the US this is important for determining eligibility for voting in primaries.
Only people on the electoral register can vote - and unless you have a postal vote - you may only vote at the specified polling station (which is why it is important to advise the Electoral Registration Officer of a change of address).
For further information you can access the Electoral Commission's website about the electoral register here.
If you haven't yet registered go to this site now. Tuesday 20th April is the last day!!!