Cost saving plus some increase in security.
I've no idea whether the Deputy Prime Minister is still looking at this site. Despite the promised frequent updates, nothing has been heard since 9 July. But just in case………
Each year an electoral canvass is required by law. Forms for completion are delivered to all households which are, or might be, residential. "No change" returns can be done online or by phone. To satisfy the law all others must bear a valid signature. This means processing (and presumably storing) many thousands, even millions, of forms. Reminders must be sent, canvassers must call. A colossal cost.
If the law were changed so that only an initial A5 page was delivered containing a code, then internet returns on a pro forma could supply many of the details, which should then self populate the new register. Spot checks would still be required but manual input would be much reduced.
A phone number would be required for those who preferred the form (and didn't want to download it) plus some canvassing input for "no shows". Substantive telephone returns could not be made due to the risk of error.