I think you raised this question before, Dr F and I think I provided the answer. However, I may be wrong, so, the Representation of the People (England and Wales) Regulations 2001 makes it mandatory:
Power to require information
23. (1) A registration officer may require any person to give information required for the purposes of that officer’s duties in maintaining registers of parliamentary and local government electors.
(2) A registration officer is under a duty to require persons to give information required for the purposes of that officer’s duty under section 3(1) of the Juries Act 1974(1).
(3) If any person—
(a)fails to comply with, or
(b)gives false information in pursuance of,
any such requisition of the registration officer as is mentioned in this regulation, he shall be liable on summary conviction to a fine not exceeding level 3 on the standard scale.
You will note that the matter of whether one intends to vote or not is not relevant. You should also note that the register is used to call people for jury duty. Full details here:
http://www.legislation.gov.uk/uksi/2001/341/part/III/made