Either a local council or a firm which wants to send out junk mail.
The law requires every eligible adult, resident in the UK, to register on the electoral roll (which is frequently misspelt as 'role' by the way!). The full version of the electoral roll (i.e. with all voters names on it is compiled by each local council (using data entry staff to do so). It's a public document but, for data protection purposes, can only be consulted in person at local council offices (or sometimes in libraries and post offices). The listings are only in street order. (i.e. if you wanted to look for 'Freda Jones' you'd have to look through every street in the area until you found her name).
However when people register for the (full) electoral roll they're asked to indicate whether they want their name to appear on the edited electoral roll. (That's the same as the full roll but with the names of people who don't want to be found, or simply who don't like junk mail, deleted from it). Councils can sell the edited electoral roll to anyone who wants to buy it. The roll is searchable by name so, in order to find Freda Jones someone can simply enter her name and find her address. Such electoral rolls cost thousands of pounds, so firms like 192.com buy them and then sell on the data on a subscription basis to, for example, people who're trying to find family members.
Similarly, firms who compile mailing lists for companies who want to send out advertising mailshots buy the edited electoral rolls to select their mail recipients from. They might also use data entry staff in order to create such mailing lists from the edited roll.
Chris