Until 2003 there was only one register, and it was REQUIRED by law to be sold to anybody who asked. Once upon a time this didn't matter much - it was only available on paper. Later it was available as a computer file, and thus good for sending junk mail.
There were a lot of complaints, over many years, until eventually the edited register was introduced. This could still be sold to anybody, but you could tick a box to not be on it. Junk mailers still used the 2002 list, but this is gradually getting so flakey that it may not be any use for much longer.
Credit reference agencies still get the full version, but it is an offence (specific, not just DPA) for them to sell or give it to anybody else. The low value of the list would not, in any case, be worth the risk to their business model.
On a side note, before the individual registration issue, before it came in a letter would arrive, addressed to "The Householder" saying that it was a criminal offence for said householder not to fill in the form. I contacted our Electoral Registration Officer to ask who the would prosecute in such case, and was told either the owner, or the tenant who had signed the rental. I pointed out that our house was in joint names, and over recent years we had taken it in turns to sign the form - so who would be prosecuted. He had no answer, and said that they had never actually prosecuted anyone anyway.