The (fee-paying) online sites only provide access to the 'open' electoral register, which excludes every voter who has ticked the box on their registration form to say that they don't want their details to appear on the open version. (The open electoral register can be bought by anyone, such as direct mail companies).
Data protection laws mean that the full version of the electoral roll can only be inspected at local council offices and under the supervision of a council official. (In many cases that can mean that you need to make an appointment to view the register, to ensure that someone is available to supervise your viewing). You're not permitted to make any form of copy of the register, so you can't photograph it with your mobile phone or even make handwritten notes from it. You'll also be required to sign a form declaring that you won't use the information in any prohibited manner. (See 'Full version' here:
https://www.babergh.gov.uk/elections/register-of-electors/ )